Twinkle twinkle little star – How I wonder what you are !


Astronomy – a simple science?

“Simplicity is the ultimate sophistication.” – Leonardo da Vinci.

Astronomy may not sound like a very simple science. But in reality, many things in astronomy require only basic fundamentals that we all learn in high school. The sky is a natural laboratory for astronomers and fundamentals of physics are used to study the properties of objects in space. To give a demonstration of this, we will take a look at how simple it is to get a rough estimate of the properties of a star which is probably billions of light years away from us.

How do we know about the stars

Even an astronomer having access to state-of-the art technology can obtain values of only two variables – apparent brightness of the star and it’s distance from us.

Apparent brightness and spectrum[1] can be measured and recorded by a variety of light sensitive devices. The distance can be obtained from simple methods like parallax. To demonstrate parallax, hold your index finger at eye level. Shut your left eyelid first (right eyelid open) and look at your finger, then shut your right eyelid and look at the finger. Your finger will appear to move against the backdrop of a more distant object such as the wall of your study room. This apparent motion is called parallax. Astronomers use a similar technique to find distance to the star. Astronomers make one observation each two times a year, when the Earth is on the opposite sides of the sun.

Now let’s go from what we can measure to what we can infer and calculate. We will have a look at how a star’s three most important properties – Luminosity, Surface temperature and the radius – are calculated. Brightness, spectra and distance can be used to obtain these properties.

Let’s first learn a little bit about these quantities and why we are interested in them. Luminosity is an important measure of brightness, which is the power of a star — the amount of energy (light) that a star emits from its surface. It is usually measured in terms of how much more/less bright a star is as compared to the sun. Surface temperature of a star is used for classifying the star into different types. The radius of a star is very important from the point of view of assessing the star’s evolutionary phases[2]. The equations for these quantities are what we learnt in high school or early college years. Let’s get a recap of those:

Equation for Luminosity:
L= 4πD2b
L= Luminosity of the Star
D = Distance of the star from Earth
b = Apparent Brightness
Equation for Surface temperature:
T =  0.0029  ⁄ λmax   
T = Surface Temperature
λmax = Maximum wavelength from spectra
Equation for Radius of Star:
L = σT44πR2
R= Radius of the Star

We already have the values of the inputs (D, b, λmax ). So calculating the outputs (L, T, R) is an easy task.

Astronomers: We work hard too!

Well, now you might say if stellar astronomy is just about three simple equations, then why are astronomers spending hours on research and taking thousands of measurements using precious telescope time? Well, it turns out that getting the initial measurements is not so simple.

When we look at the sky, the stars appear like they are distributed on a huge 2D black canvas. But in reality, there is space between us and the stars. Therefore it’s not uncommon that our view of the stars may be obstructed by some interstellar gas or dust cloud. When starlight passes through this interstellar cloud before reaching earth its spectra will get distorted. Now it’s a challenge to separate which part of the spectrum comes from the stars and which one from the interstellar cloud. This is a real problem and requires several observations. There are various techniques that astronomers use to separate the two. But all of them rely upon using what we know about the stars to infer which parts of spectrum belong to the interstellar cloud. After that astronomers can subtract that part of the spectrum to give them the spectrum of the stars. Once astronomers have learnt about the interstellar spectrum they can use this to correct spectra of other stars in that direction.

Overall, we can see that although the fundamentals are simple to grasp, the practical challenges in astronomy are immense and require a lot of ingenuity and also dedication to the goal of advancing our knowledge about the universe. And in our opinion, that’s what makes it so challenging.

[1] When astronomers split the light beam coming from a star using a spectrograph (prism like device), the resulting collection of wavelengths of light is called a spectrum.
[2] A star goes through various evolutionary phases starting from a big gas cloud to a huge supernova explosion.  Right now, our Sun is in a phase called the ‘Main Sequence’ which is roughly in the middle of the two aforementioned limits.

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