All About Astronomy

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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.


A Pool of Distant Galaxies. Credit: ESO

Dive right in to this image that contains a sea of distant galaxies! The Very Large Telescope has obtained the deepest ground-based image in the ultraviolet band, and here, you can see this patch of the sky is almost completely covered by galaxies, each one, like our own Milky Way galaxy, and home of hundreds of billions of stars. A few notable things about this image: galaxies were detected that are a billion times fainter than the unaided eye can see, and also in colors not directly observable by the human eye. In this image, a large number of new galaxies were discovered that are so far away that they are seen as they were when the Universe was only 2 billion years old! Also…

This image contains more than 27 million pixels and is the result of 55 hours of observation, made primarily with the Visible Multi Object Spectrograph (VIMOS) instrument. To get the full glory of this image, here’s where you can download the full resolution version. It’s worth the wait while it downloads. Or click here to be able to zoom around the image.

In this sea of galaxies – or island universes as they are sometimes called – only a very few stars belonging to the Milky Way are seen. One of them is so close that it moves very fast on the sky. This “high proper motion star” is visible to the left of the second brightest star in the image. It appears as a funny elongated rainbow because the star moved while the data were being taken in the different filters over several years.

The VLT folks describe this image as a “uniquely beautiful patchwork image, with its myriad of brightly coloured galaxies.” It shows the Chandra Deep Field South (CDF-S), one of the most observed and best studied regions in the entire sky. The CDF-S is one of the two regions selected as part of the Great Observatories Origins Deep Survey (GOODS), an effort of the worldwide astronomical community that unites the deepest observations from ground- and space-based facilities at all wavelengths from X-ray to radio. Its primary purpose is to provide astronomers with the most sensitive census of the distant Universe to assist in their study of the formation and evolution of galaxies.

The image encompasses 40 hours of observations with the VLT, just staring at the same region of the sky. The VIMOS R-band image was obtained co-adding a large number of archival images totaling 15 hours of exposure.

Source: ESO
Cited from : universetoday

Even though I am an undergraduate chemical engineering student, I have another hobby in my spare time. That is astronomy. Maybe it has no association with my study now but this is something that keeps my interest since I was in the high school.

Many people are surprised when they know I’m interested in astronomy. Maybe because astronomy is kind of unusual hobby. Not so many people are interested in astronomy, especially learning astronomy not just as a popular science but deeper than that.

The history of this hobby started when I was participated in 2004 National Astronomy Olympiad, which is also a part of the 4th National Science Olympiad. First, it was a total blur. No information, no guidance were given to me and my friends when we were told that we passed the province selection and will represented DKI Jakarta. We tried to find as much information about astronomy from the internet in school.

Honestly, we had no guess about what we had to learn. Finally, I managed to win a silver medal and one of my schoolmates won a gold medal. Surely, it was a total surprise for both of us. After a couple of week, both of us returned to Bandung to participate in the training program to give us material we need to learn to participate in International Astronomy Olympiad and also to select few of us to be the team that will represented Indonesia in that competition. Unfortunately, both of us weren’t selected.

Back to school, we felt that we had to initiate the development of astronomy in our school. So, we suggested our head principal to found the astronomy club. It is a club which students are given material of astronomy. We used our knowledge we had from training program to help our teacher to give lessons about astronomy. I also made a book which compiled all the material of astronomy in a book titled “Modul Persiapan Menuju Olimpiade Astronomi” as a source for the next astronomy olympiad preparation.

After 2 years I become a chemical engineering student in ITB, Bandung, I finally decided to make a blog about astronomy. I hope it can help other students to learn astronomy. So, no students will experience the problems I’ve ever had back in those very first year involving in astronomy Olympiad.

If you also interested in learning astronomy, you can visit my blog.
Hopefully, the astronomy awareness will grow in the society, especially to welcome the International Year of Astronomy in 2009.

Hello everybody. This is my first post for this new blog. I released this blog as a support to my other astronomy related blog : belajar Astronomy. All about astronomy is intended for everybody who has interest in astronomy. The difference with my other blog (belajar Astronomy) is the main language used. This blog will use English as the main language. The reason is English is one of the world’s most recognized language, so everybody can read and understand this blog well. Informations, materials, news and other things will be published to increase your knowledge in astronomy. So, I hope all the people who interested in astronomy can take some benefits from this blog.

(visit also my other blog but it is not astronomy-oriented blog : Free Your Mind)

October 2020



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