All About Astronomy

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belajar Astronomy

Looking through a telescope at the stars there is very little information we can gain from them. To be sure, we know what color they are and we can see that some are more luminous than others. If we use a spectrograph we can tell what elements they are made up from. From these facts alone, it is difficult to tell just how much mass they contain.

By looking at pairs of stars that orbit one another we can try to answer the question, how much mass do the stars have?

Binary stars can be of two fundamental types:

  • Visual Binaries
  • Optical Doubles
Alberio (Visual Binary)
Visual Binaries are stars that are clearly gravitational associated with one another. They orbit each other around a common center called the barycenter. Visual binaries can be seen optically through a telescope. Only a small portion of binary stars are visual binaries. In order to see a visual binary, the stars must be separated by fairly wide distances, and the orbital periods are usually very long.

Optical Doubles are stars that appear to lie close together, but in fact do not, they only appear to us from our earthly observation to be close together. One of the stars in the pair is actually behind the first star and very far away. The stars of an optical double are not gravitationally bound.

William Herschel began looking for optical doubles in 1782 with the hope that he would find a measurable parallax, by comparing a close star to the more distant star in an optical double.
Herschel did not find any optical binaries, but he did catalog hundreds of visual binaries. In 1804 Herschel had so many measurements of visual binaries that he concluded that a pair of stars known as Castor were orbiting one another. This was an important discovery, because it was the first time observational evidence clearly showed two objects in orbit around each other outside of the influence of our own Sun and Solar System.

Spectroscopic Binary
It is also possible to detect binary stars using a spectroscope. If two stars are orbiting each other they will both produce a spectrum. If the stars are close to being the same brightness it is possible to see different spectral lines from both stars. These stars are of particular interest because it can be used to determine the radial velocity of the orbit of the two stars. Stars appear red shifted when receding away from the earth and blue shifted as they approach. This effect is caused by the Doppler effect which distorts arriving light waves from the stars depending on the direction if their motion. A Spectroscopic binary will alternate between blue and red shifted spectral lines.

Spectroscopic binaries are not detectable if we are seeing the star head on because no Doppler shifts would be present in the spectrum. If the Doppler shifts are present in a single line of the spectrum, we are seeing the light from only one star and we call this a single-line spectroscopic binary. If we can see the light from both stars the Doppler shifts will alternate, split and merge depending on the positions of the two stars in their orbits. This is called a double-line spectroscopic binary.

One very important detail, we do not know how the orbits of the two stars are inclined to earth. This inclination could be any angle, for that bit of information we have to go back to visual methods in order to see the individual stars to determine the inclination of their orbits relative to earth. Even so we can not for certain determine the true inclination of the orbit so our mass calculation is only a lower limit to the masses of the two stars.

Radial velocities permit astronomers to compute the total mass for the two stars, they do not provide the masses for the individual stars and other methods must be used to make that determination

Eclipsing Binary
Another type of binary called the Eclipsing binary can be studied. The information gathered can be used to calculate the individual stellar masses and the diameters of the individual stars. It is rare to find two stars in orbit around one another to have orbital inclination where the stars pass in front of one another to form one point of light as seen from earth.
When the orbital inclination if the eclipsing binary is edge on to earth, the stars will seem to pass in front of one another as they orbit, when the light from the brighter star is eclipsed we will see a deep decline in the amount of light received from the star (6/25/95 in Figure 1) we call this primary minimum, also when the light from the dimmer star is blocked by the brighter the light received declines again, but not so deep and we call this secondary minimum (see 6/9/95 in Figure 1) , otherwise we are able to collect some or all of the light from both stars.

The pattern of these light changes is called a light curve and the data for it gathered by the use of a photometer, making periodic measurements until the eclipsing binaries produce a complete orbital cycle.

We use the mass vs. luminosity relationship to determine what the difference is between the individual masses, then using the mass of the entire system calculated from the radial velocity information, we can determine what the individual masses of the two stars should be. The photometeric data removes some of the uncertainty in regard to the inclination because the shapes of the light curves will be different for a partial eclipse than for a total eclipse.
ALGOL is one of the best known and most studied eclipsing binary stars. ALGOL is normally about 2.3 magnitude, but every 10 hours or so it will dim to about 3.4 magnitude, in other words ALGOL becomes 68% dimmer. I suspect that humanity has known about ALGOL’s behavior for quite some time, since the Arabic name of ALGOL means “Demons Head”, and ALGOL is associated with the severed head of Medusa. ALGOL is often referred to as the winking eye of the demon.

An eclipsing binary occurs when the orbital plane of the binary system is exactly When one star passes directly in front of the other, as viewed from Earth, we seen an eclipsing binary perpendicular to the plane of the sky.

Dwarf Nova or Recurrent Nova

When an otherwise normal star is associated with a white dwarf companion, a type of binary called a recurrent nova, or dwarf nova may occur. The normal star transfers mass onto an accretion disk which forms around the white dwarf. As material falls onto the accretion disk some of the material may be transferred to the white dwarf by turbulence in the accretion disk, this causes a sudden brightening of the white dwarf as the hydrogen is converted into helium.
If enough material from the accretion disk falls onto the white dwarf the hydrogen gas will become compressed and will not immediately fuse until a substantial increase in temperature occurs; the material will suddenly and violently erupt fusing into a runaway fusion reaction and a violent eruption called a dwarf nova occurs which will blow the accretion disk away, but it will not disturb the normal star.

Mass transfer will quickly resume and a new accretion disk will form. The cycle will continue until enough mass is drawn off the normal star to halt the reaction.

Mass transfer in any type of binary system will affect the evolutionary cycle of the two stars. The normal star will burn its fuel more slowly as mass is removed and the star cools down due to less internal heating from gravitational forces. It will also accelerate the evolution of the star receiving the mass, for the same reasons, more mass, more internal heating and the hastening of the fusion process.

If the material transfers very quickly, the gravitational forces will prevent the hydrogen from fusing by compressing it even further until the hydrogen gas becomes degenerate matter. Degenerate matter does not expand due to the increases in temperature so the mass of the white dwarf increases until it exceeds the Chandrasekhar Limit. When this happens the white dwarf will collapse and a type I supernova will occur which may destroy the companion star and the white dwarf changes into a neutron star or a black hole.

A similar event can occur when a normal star is associated with a pulsar, the energy given off will be mostly X-rays however, and instead of being called a dwarf nova or recurrent nova, it is called an X-ray burster or more simply a burster. We think that as normal hydrogen falls onto the accretion disk it is quickly converted into helium, when the helium reaches a depth of 1 meter, it will explosively convert helium into carbon producing X-rays. The longer the delay in fusing carbon, the larger and more violent the burst will be. The main difference between the recurrent nova and the burster is that the accretion disk will be hotter in the burster because it is already fusing hydrogen into helium, also the burst will produce mostly X-rays instead of visible light.

When a black hole is associated with a normal star, it will produce the same events as an X-ray burster and the only way to be sure that the companion is a blackhole, is when the mass of the compact object is greater than 3 solar masses. This is far too much mass for the companion to be a neutron star. The gravitational forces would cause the collapse of the star beyond the point of the neutrons to support themselves against the force of gravity and the star would collapse to a zero radius creating a black hole.

Calculation of star’s properties with binary stars
Types of Binaries

  • Visual Binary: Can see both stars and follow their orbits over time.
  • Spectroscopic Binary: Cannot separate the two stars, but see their orbit motions as Doppler shifts in their spectral lines.
  • Eclipsing Binary: Can separate the stars, but see the total brightness drop when they periodically eclipse each other.

Visual Binaries –> Two stars orbiting about their center-of-mass.

Center of Mass
Two stars orbit about their center of mass.

  • Measure semi-major axis, a, from projected orbit & the distance.
  • Relative positions about the center give: M1/M2 = a2/a1

Measuring Masses
Newton’s Form of Kepler’s Third Law:
1. Measure the period, P, by following the orbit.
2. Measure semi-major axis, a, and the Mass Ratio, M1/M2, from the projected orbit on the sky.
3. Solve the equation above and separate Masses.

We need to follow an orbit long enough to trace it out in detail:

  • This can take decades
  • Need to work out the projection on the sky

Measurements depend on knowing the distance:

  • semi-major axis depends on d
  • derived mass depends on d^3

Small errors add up quickly (10% error in distance translates into a 30% error in the mass!).

Spectroscopic Binaries
Most binaries are too far away to be able to see both stars separately.
But, you can detect their orbital motions by the periodic Doppler shifts of the spectral lines:

• Determine the orbit period & size from the pattern of orbital velocities

Cannot see the two stars separately:

  • Semi-major axis must be guessed from the orbit motions.
  • Can’t tell how the orbit is tilted on the sky

Everything depends critically on knowing the distance.

Eclipsing Binaries
Two stars orbiting nearly edge-on to our line-of-sight.

  • See a periodic drop in brightness as one star eclipses the other.
  • Combine with spectra which measure orbital speeds

With the best data, one can find the masses of the stars without having to know the distance!!!

Eclipsing Binary stars are very rare.
Measurement of the light curves is complicated by details:

  • Partial eclipses yield less accurate numbers.
  • The atmospheres of the stars soften the edges.
  • Close binaries can be tidally distorted.

However, the best masses are from eclipsing binaries.

Source : many different sites

It’s been long time since the last time I posted a trivia question. This one is an easy one. Can you mention the name of the object?


Klaudia Einhorn.

COROT-exo-7b, bottom left dot shadows in front of his central star (artist's impression). Because of its proximity to large solar researchers suspect temperatures over 1000 degrees Celsius on the extrasolar planets. Image: Klaudia Einhorn.

The CoRoT satellite has found the smallest terrestrial exoplanet yet, — with a diameter just under twice that of Earth — complete with a rocky surface you could walk on and possibly even oceans to sail across. However, if you traveled there, you might want to bring some protection, as the temperature of this planet is likely very high. CoRoT-Exo-7b is located very close to its parent star, orbiting once every 20 hours. Astronomers estimate temperatures on the planet could be between 1000 and 1500°C and it possibly could be covered in lava or water vapor. This latest exoplanet was detected as it transited in front of its parent star, dimming the light from the star just enough to be noticeable.

The parent star lies about 140 parsecs from Earth, located about half way between the star Sirius in Canis Major and Betelgeuse, the red giant star in Orion.

The internal structure of CoRoT-exo-7b particularly puzzles scientists; they are unsure whether it is an ‘ocean planet’, a kind of planet whose existence has never been proved so far. In theory, such planets would initially be covered partially in ice and they would later drift towards their star, with the ice melting to cover it in liquid.


COROT detects small, transiting exoplanet. Credits: CNES

“This discovery is a very important step on the road to understanding the formation and evolution of our planet,” said Malcolm Fridlund, ESA’s CoRoT Project Scientist. “For the first time, we have unambiguously detected a planet that is ‘rocky’ in the same sense as our own Earth. We now have to understand this object further to put it into context, and continue our search for smaller, more Earth-like objects with COROT,” he added.

About 330 exoplanets have been discovered so far, most of which are gas giants like Jupiter and Neptune. The density of COROT-Exo-7b is still under investigation: it may be rocky like Earth and covered in liquid lava. It may also belong to a class of planets that are thought to be made up of water and rock in almost equal amounts. Given the high temperatures measured, the planet would be a very hot and humid place.

transiting-movie“Finding such a small planet was not a complete surprise”, said Daniel Rouan, researcher at the Observatoire de Paris Lesia, who coordinates the project with Alain Léger, from Institut d’Astrophysique Spatiale (Paris, France). “CoRoT-Exo-7b belongs to a class of objects whose existence had been predicted for some time. COROT was designed precisely in the hope of discovering some of these objects,” he added.

Small terrestrial planets are difficult to detect, and so very few exoplanets found so far have a mass comparable to Earth, Venus, Mars, and Mercury. Most of the methods used to find planets are indirect and sensitive to the mass of the planet. The CoRoT spacecraft can directly measure the size of a planet’s surface, which is an advantage. In addition, its location in space allows for longer periods of uninterrupted observation than from ground.

Astronomers say this discovery is significant because recent measurements have indicated the existence of planets of small masses but their size remained undetermined until now. CoRoT (Convection Rotation and Transits) was launched in December 2006 and consists of a 27 cm-diameter telescope designed to detect tiny changes the brightness of nearby stars. The mission’s main objectives are to search for exoplanets and to study stellar interiors.

Source: ESA
Cited from : Universe Today by Nancy Atkinson

I posted here some exercise problems that you can use to expand your knowledge in some chapter in basic astronomy :

1.)  The following statements are true or false:

  • (a) Of all the natural satellites in the Solar System only the Moon always turns the same face towards its primary.
  • (b) The mass of a planet in the Solar System can be determined only if it possesses one or more satellites.
  • (c) The planet with the largest apparent angular diameter when nearest the Earth is Venus.
  • (d) Pluto is the planet farthest from the Sun.
  • (e) A lunar eclipse may occur if the Moon is new.

2.) Calculate the mean density of Jupiter from the following data, assuming the orbits of Earth and Jupiter to be circular and coplanar:

  • Angular semi-diameter of Jupiter at opposition = 21”,8
  • Orbital radius of Jupiter = 5,2 A.U.
  • Mass of Jupiter/mass of Earth = 318
  • Mean density of Earth = 5,5 kg/m^-3
  • Sun’s horizontal parallax = 8”,8

3.) The two components of a binary star are approximately equal brightness. Their maximum separation is 1”,3 and the period is 50,2 years. The composite spectrum shows double lines with a maximum separation of 0,18 Angstrom at 5000 Angstrom. Assuming that the plane of the orbit contains the line of sight, calculate (i) the total mass of the system in the terms of the solar mass, (ii) the parallax of the system.

=================================================== (source : Astronomy, The Structure Of Universe).

Spectroscopy is a branch study in astronomy that focus on astronomical objects’ spectrum. From the spectrum, we can get informations, such as its temperatures, chemical compositions, movement speed, etc. That’s why spectroscopy can be considered as one of the fundamental field in astronomy. The spectrum of a star (or any other astronomical object) is acquired by using an instrument called spectrograph.

Fig 1. Spectrum

Fig 2. Spectrograph

One of the fundamental law in spectroscopy is Kirchoff Law (1859) which stated that:

  1. If a liquid or high pressure gas is ignited, they will emit energy in all wavelength which will produce a continuous spectrum.
  2. If a low temperature gas is ignited, it will only emit energy in certain range wavelength and produce spectrum which have a dark background and some bright lines. That kind of spectrum is called the emission spectrum. The wavelength of each bright lines are the precise indicator of what gas that produce them. So, the same gas will produce bright lines in certain exact wavelength.
  3. If a white light (which is a equal mixture of all colors) is passed through a cool low temperature gas, the gas will absorb energy at certain wavelength. The result spectrum will be continuous spectrum as the background with some dark lines in certain exact wavelength. The dark lines called absorption lines and that kind of spectrum is called the absorption spectrum. The wavelength of each dark lines are the precise indicator of what gas that produce them. So, the same gas will produce dark lines in certain exact wavelength.
Fig. 3 & 4. Continuous, emission and absorption spectrum (respectively)

Balmer Series
Switzerland scientist, Balmer, state a series equation to predict the wavelength of the absorption lines of hydrogen gas. The equation is widely known as Balmer series equation.

with : λ: the wavelength of the absorption lines [cm]
RH : Rydberg constant (= 109678 )

Fig. 5 : Emission spectrum of hydrogen that exhibit the first four emission lines in Balmer’s series

Planck’s Quantum Law

Planck postulates that light is radiated in the form of small discrete package called quantum. This theory is the foundation of the birth of a new field in physics called quantum physics.

Planck state that energy of each photon

Eo = h. f = hc//λ

h : Planck’s constant (h = 6,63 x 10^-34 J.s)
f : frequency of the photon [Hz]
c = speed of light (= 3.10^5 km/s)
λ = photon’s wavelength

Star’s spectrum
Star’s spectrum pattern is wide in variety. In 1863, an astronomer called Angelo Secchi classified star’s spectrum in 4 groups based on the similarities of its’ absorption lines.

Miss A. Maury from Harvard Observatory establish another way to classify star’s spectrum and it was revised by Miss Annie J. Cannon. Miss Cannon’s classification is the most widely adopted today.

Table 1 : Resume of the classification of star’s spectrum (to remember it use the donkey bridge : Oh Be A Fine Girl (or Guy), Kiss Me). (you can click the figure to get bigger and clearer version of the table above; .

Sub-classification of star’s spectrum
Star’s spectrum classification O, B, A, F, G, K, M is divided again to several sub-classes :
B0, B1, B2, B3, . . . . . . . . ., B9
A0, A1, A2, A3, . . . . . . . . ., A9
F0, F1, F2, F3, . . . . . . . . . ., F9

Bigger number represent lower temperature! The use of this sub-class is to narrow the specification’s range and become more precise.
(for further information, check this site.)

Fig 6. Star’s spectrum from different classes

M-K Classification (Star’s Luminosity Class)

Stars with same certain spectrum’s class is found to have different luminosities. In 1913, Adam dan Kohlscutter from Mount Wilson Observatory showed that the width of spectrum’s lines can be used to estimate star’s luminosity.
Based on these facts. in 1943 Morgan and Keenan from Yerkes Observatory divided stars to several luminosity class as shown in the table below.


Very bright super giant star

Class 1b

Less bright super giant star

Class II

Bright giant star

Class III

Giant star

Class IV

Sub-giant star

Class V

Main sequence star

Table 2. Morgan Keenan’s Luminosity Class

Morgan Keenan’s Luminosity Class (M-K class) is sketched in a Hertzprung-Russell diagram (H-R diagram) below.

Fig 7. Star with different luminosity class in a H-R diagram

Now, star’s classifications use the combination of spectrum class and luminosity class. For example : A G2 V star is a main sequence star that belongs to spectrum class G2

Star’s motion
Contrary to widely beliefs that star isn’t moving in space, star DO move in space. However, the movement of stars is hard to track. Beause of its immense distance, the movement of star only produce extremely small apparent movement in sky. We have to wait several years (or decades!) to track star’s movement in sky. Warning : the star’s movement that is discussed above is not the apparent daily motion of the star !

The star’s angular motion of a star is called proper motion (μ). Proper motion is usually measured in arc-second per year. Star with biggest proper motion is Barnard Star with μ = 10”,25 per year (In 180 years, this star will (only) move in extent as full Moon’s disk).

Fig 8. Star’s motion

Relationship between tangential velocity (Vt) and the proper motion (μ):

Vt = 4,74 μ d

with :

Vt = tangential speed of the star [km/s]

μ = proper motion of the star [“/ year]

d = star’s distance [parsec]

the above equation also can be stated as :

Vt = 4,74 μ/p

with p is the parallax of the star (in arc second).

The proper motion is measured by two quantities: the position angle and the proper motion itself. The first quantity indicates the direction of the proper motion on the celestial sphere (with 0 degrees meaning the motion due north, 90 degrees due east, and so on), and the second quantity gives the motion’s magnitude, in seconds of arc per year.

Fig 9. Star’s proper motion

The equations used to find the quantity of star’s proper motion are :

μα cos δ = μ sin θ
μδ = μ cos θ

with :
μα = proper motion in right ascension
μδ = proper motion in declination
μα and μδ is measurable –> μ and θ can be determined.

Beside proper motion, information about star’s motion can be obtained from its radial motion, which is the component of star’s motion that lies parallel to our line of sight.
Radial velocity (Vr) can be measured by its spectrum lines that shift (Doppler shift). For star which radial velocity (Vr) is significant compared to the speed of light:

For Vr being much smaller compared to the speed of light (c), the equation can be simplified to:

Δλ/λo = Vr/c

with :
Δλ = the difference between static wavelength (λo) and observed wavelength (λ). [Å or nm]
λo = static wavelength. [Å or nm]
Vr = radial velocity [km/s]
c = speed of light (300.000 km/s )

Fig 10. Red shift and blue shift

Now, we are able to calculate Vt and Vr as discussed above and we will be able to calculate star’s true motion (linear motion):

V2 = (Vt)2 + (Vr)2


  1. “Astrofisika I” lecture notes, by Dr. Djoni N. Dawanas
  2. Wikipedia
  3. Google images

For other pages that discuss this material, you are advised to visit these sites:
1. Spectroscopy


The International Year of Astronomy 2009 is a global effort initiated by the International Astronomical Union and UNESCO to help the citizens of the world rediscover their place in the Universe through the day- and night-time sky, and thereby engage a personal sense of wonder and discovery.

SGAC is pleased to announce an essay competition.

“What does astronomy bring to civil society?”

We are looking for Young Astronomers who can express their feelings in a creative way. 2 first prize will be invited to Paris to participate “Launch Conference of the International Year of Astronomy” between 14-18 Jan 2009. Accommodation and travel will be covered by UNESCO and IYA French National Committee.

Submission Procedures , Timeline and Criteria

Eligibility Criteria

* Authors should be between 18-23 years old


  1. Essays can be a maximum of 1500 words. In addition, an abstract of a maximum of 200 words is required and should be included with the submission.
  2. Essays must be submitted in English. No high level spell check needed for authors whose first language is not English.
  3. Essays must be typed on Times New Roman 11 and double-spaced and should be delivered in .doc or .pdf format.
  4. Essays should be in a form readily publishable without extensive footnotes.

The selection committie will be guided by the following criteria:

  • Inspiration on Astronomy
  • Innovation & originality
  • Relevance to IYA2009
  • Quality on writing style and structure


Essays should be delivered by e-mail to 12 December 2008, 17:00 GMT.

Clear Skies….

Halit & Bee
SGAC IYA09 Co-Liaisons

from :

So, let’s take a part.

Spitzer, Chandra and Calar Alto Telescope

Tycho's Supernova Remnant. Credit : Spitzer, Chandra and Calar Alto Telescope

On November 11, 1572 Danish astronomer Tycho Brahe and other skywatchers observed what they thought was a new star. A bright object appeared in the constellation Cassiopeia, outshining even Venus, and it stayed there for several months until it faded from view. What Brahe actually saw was a supernova, a rare event where the violent death of a star sends out an extremely bright outburst of light and energy. The remains of this event can still be seen today as Tycho’s supernova remnant. Recently, a group of astronomers used the Subaru Telescope to attempt a type of time travel by observing the same light that Brahe saw back in the 16th century. They looked at ‘light echoes’ from the event in an effort to learn more about the ancient supernova.

A ‘light echo’ is light from the original supernova event that bounces off dust particles in surrounding interstellar clouds and reaches Earth many years after the direct light passes by; in this case, 436 years ago. This same team used similar methods to uncover the origin of supernova remnant Cassiopeia A in 2007. Lead project astronomer at Subaru, Dr. Tomonori Usuda, said “using light echoes in supernova remnants is time-traveling in a way, in that it allows us to go back hundreds of years to observe the first light from a supernova event. We got to relive a significant historical moment and see it as famed astronomer Tycho Brahe did hundreds of years ago. More importantly, we get to see how a supernova in our own galaxy behaves from its origin.”

Subaru Telescope

The view of the light echoes from Tycho’s supernova. Credit: Subaru Telescope

On September 24, 2008, using the Faint Object Camera and Spectrograph (FOCAS) instrument at Subaru, astronomers looked at the signatures of the light echoes to see the spectra that were present when Supernova 1572 exploded. They were able to obtain information about the nature of the original blast, and determine its origin and exact type, and relate that information to what we see from its remnant today. They also studied the explosion mechanism. What they discovered is that Supernova 1572 was very typical of a Type Ia supernova. In comparing this supernova with other Type Ia supernovae outside our galaxy, they were able to show that Tycho’s supernova belongs to the majority class of Normal Type Ia, and, therefore, is now the first confirmed and precisely classified supernova in our galaxy. This finding is significant because Type Ia supernovae are the primary source of heavy elements in the Universe, and play an important role as cosmological distance indicators, serving as ‘standard candles’ because the level of the luminosity is always the same for this type of supernova. For Type Ia supernovae, a white dwarf star in a close binary system is the typical source, and as the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst. However, as Type Ia supernovae with luminosity brighter/fainter than standard ones have been reported recently, the understanding of the supernova outburst mechanism has come under debate. In order to explain the diversity of the Type Ia supernovae, the Subaru team studied the outburst mechanisms in detail. This observational study at Subaru established how light echoes can be used in a spectroscopic manner to study supernovae outburst that occurred hundreds of years ago. The light echoes, when observed at different position angles from the source, enabled the team to look at the supernova in a three dimensional view. This study indicated Tycho’s supernova was an aspherical/nonsymmetrical explostion. For the future, this 3D aspect will accelerate the study of the outburst mechanism of supernova based on their spatial structure, which, to date, has been impossible with distant supernovae in galaxies outside the Milky Way.

The results of this study appear in the 4 December 2008 issue of the science journal Nature.

Source: Subaru Telescope

Cited from : universe today by Nancy Atkinson

An ilustration of the Christmas star sightings

An ilustration of the Christmas star sightings

‘This is the season… And every year around this time people notice the brilliant ‘star’ to the west just after sunset. For astronomers, we know it’s the appearance of the planet Venus, but noticing it for the average person brings on questions about the holidays. Was the Christmas Star real?

Regardless of your faith, the story of the ‘Star of Bethlehem’ is one of the most powerful and enigmatic symbols of Christianity. For centuries, scientists, scholars and historians have debated about the nature of this biblical light that heralded an event. Was it purely a divine sign, created miraculously to mark a special birth? Or was it an astronomical event in its own right?

David Reneke, news editor of Australia’s Sky and Space Magazine, believes astronomers may have found the answer – or at least something that fits all the known facts – basing his research on the highly esteemed gospel according to Matthew, the first of the four gospels in the New Testament. It would appear to be the first written and this version places key players together in the same time period. “It’s generally accepted by most researchers that Christ was born between 3 BC and 1 AD.” says Dave. With the aid of modern astronomy software programs astronomers can reproduce the night sky exactly as it was, thousands of years ago. Humans are curious and so was Dave, so he turned back the hands of time and the stars to the time of that long ago Christmas…

“We found out something startling.” said Reneke, “It looks like the ‘Christmas star’ really did exist,”

Two thousand years ago, astronomy and astrology were considered one and the same. The motions of the heavenly bodies were used to determine the events of history, and the fate of people’s lives. Of the various groups of priests and prophets of this period, those which commanded the most respect were the Magi – whose origins are not entirely clear. Known as ‘wise men’ , we can only assume they were actually priests who relied on their knowledge of astronomy/astrology.

Armed with an approximate date, Dave assumed the ‘Star of Bethlehem’ was not just a localized event and could be observed by sky-watchers elsewhere in the world, not just by the Magi. Historical records and modern-day computer simulations indicate a rare series of planetary groupings, also known as conjunctions, during the years 3 BC and 2 BC In fact, this was one of the most remarkable periods in terms of celestial events in the last 3,000 years!

“Like the final pieces of a difficult jig-saw puzzle, our fabled biblical beacon is starting to reveal itself,” David said. “On 12 August, 3 BC, Jupiter and Venus appeared very close together just before sunrise, appearing as bright morning ‘stars.’ It would have been visible in the eastern dawn sky of the Middle East from about 3:45 to 5:20 a.m.”

But it didn’t stop there. The crowning touch came ten months later, on 17 June 2 BC, as Venus and Jupiter joined up again in the constellation Leo. This time the two planets were so close that, without the use of our modern optical aids, they would have looked like one single, brilliant star. According to Dave’s research, Jupiter was known as the “planet of Kings” and Saturn as the “Protector of the Jews”. This could easily have been interpreted as a sign that the Jewish Messiah had been, or was about to be, born. Also, Leo was thought to denote royalty and power. An interpretation? Perhaps. But, do not forget the times in which this occurred. Astronomy and astrology intermingled. This whole sequence of events could have been enough for at least three astrologers to see this as sign in the heavens and make their way Jerusalem.

“Now, this doesn’t mean that astrology works,” Reneke said. “We haven’t ruled out other possibilities for the Star of Bethlehem but it does make our search more rewarding to find a truly interesting astronomical event that happened during the most likely time for the Nativity.”

Whatever the Star of Bethlehem was, it has had more impact on humankind than any star before or since. It is also possible that the mystery of the Star will never be completely solved. For many of us though, it is the mystery itself that drives us to find the solution.

Source : David Reneke

David Reneke, one of Australia’s most well known and respected amateur astronomers and lecturers, has over 40 years experience in astronomy with links to some of the world’s leading astronomical institutions. David is also the News Editor for Australia’s Sky and Space Magazine, he teaches astronomy at college level, is an invited speaker at astronomy conventions throughout Australia, a feature writer for major Australian newspapers, and is a science correspondent for ABC and commercial radio stations. In these weekly radio interviews David regularly appears on about 60 networked stations around the nation with all the latest news and on general astronomy and space discovery issues.

Cited from : universe today

Photometry is a branch field of astrophysics which learned the quantity, quality, and the direction of the electromagnetic radiation from the sky’s objects. “Photo” in photometry which means “visual light” was used because the observation was used to limited in visual light.

Photometry was based on our knowledge about radiation law. We hypothesize that the astronomical objects have characteristics of a hypothetical black body.

The characteristics of the black body are:

  1. when thermal equilibrium is achieved, the object’s temperature is a function of how many energy it absorbs per second
  2. a black body doesn’t emit radiation in all wavelength in same intensity (some emits more radiation in blue region wavelength, and the way around. The wavelength which it emits most will determine its color).

The wavelength its emits most (λmaks) by a black body which temperature is T Kelvin is :

λmaks = 0,2898/ T …………………….. (eq. 1)

(λmaks expressed in cm and T in Kelvin)

The equation 1 is called Wien’s rule.

An example of implementing the Wien’s rule :

(Warning : be clear that λmaks doesn’t mean the maximum wavelength but it means the wavelength that a body emits in the biggest intensity)

The total energy per time emitted by a black body per its surface area is called emitted energy flux. The value of the emitted energy flux from a black body with a surface temperatur T Kelvin is :

F = σT4 …………………….. (eq. 2)

(σ : Stefan-Boltzman constant: 5,67 x 10^-8 Watt/m2K4)

The total energy per unit time (= Power) that’s emitted by a black body with the surface temperature T Kelvin and surface area A is known as Luminosity. Its value (L) can be calculated by equation below:

L = A σT4 …………………….. (eq. 3)

For stars, we can assume it’s a perfect sphere. So, its surface area (A) is 4πR2 ; with R express star’s radius. So, a star’s luminosity (L) is equal to :

L = 4πR2 σT4 …………………….. (eq. 4)

he black body emits its radiation to all direction. We can assume the radiation pass through a sphere surface with a radius d in same energy flux (E).

E = L/(4πd2) …………………….. (eq. 5)

This amount of flux is received by an observer from a distance d from the black body. So, this flux is usually called received energy flux or brightness. (Warning : differ between E and F).

The equation above is often termed as the inverse square law for brightness (E) because this equation shows that brightness is inversely proportional to the square of its distance (d). So, the farther the distance, the less bright it is.

Review Questions:

  1. From an observation result, we know that an area of 1 cm2 in Earth’s outer atmosphere received Sun’s energy with intensity of 1,37 x 106 erg/cm2/s. If we know that the distance between Sun and Earth is 150 million kilometres, determine the Sun’s luminosity.
  2. Calculate the intensity of Sun’s radiation received by Saturn’s surface if we know that the distance between Saturn and Sun is 9,5 Astronomical Units (use the information from number 1)?
  3. The luminosity of a star is 100 times larger than Sun is, but the temperature of the star is only half of Sun’s temperature. Calculate the radius of the star expressed in Sun’s radius unit?
  4. Define the term Luminosity and Brightness using your own words
  5. Calculate the wavelength of maximum intensity radiation of a star which temperature is 10.000 Kelvin.
(source : Dr. Djoni N. Dawanas)
(translated from : belajar Astronomy).

Foto langit

A portion of the sky

Can you identify at least TWO constellations shown in the photograph above? Also, can you mention in what direction (north or south) this photograph was taken? You can submit the answer in the comment section.

October 2020



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