THE NIGHT SKY (Distances and Magnitudes)

The night sky is full of many wonders and mysteries. When you look at the night sky you find many glittering objects, some of which are planets, stars in our own galaxy, and even the stars in the other galaxies.

Stars-

Stars are fixed luminous points in the night sky which are large, remote incandescent bodies like the Sun.

Planets-

The word “PLANETS” originates from the GREEK word  “PLANETES”, which literally translates to “wanderers”.

Planets are the celestial bodies which revolve around their  Host stars.

DISTANCES –

•     The sky that our eyes perceive is in 2D.

•     So, it means that it’s not necessary that all celestial objects are equidistant from the observer.

•     For example-

Sun is 8 light minutes away from Earth.

Sirius (Cannis Major) is 8.611 light-years away from the Earth.

•     Units-

   light years, light minutes, light seconds, parsecs(3.2ly)

1 light year = 9.461e+12 km

HOW TO MEASURE????

Parallax -

Parallax word has its origins from the GREEK word “Parallaxis” which means “alternation”.

Parallax means the change in the apparent position of an object when measured from different points.

                 

       

As you can see in the image above, we take readings of the star (or any object) from 2 pts. i.e. we measure the position of the star (or any object) 6 months apart and hence, we measure its parallax angle.

LIGHT YEAR so large yet so small, as the compared scale of UNIVERSE….!!!!!

New UNIT introduced- parsec

                                             1 parsec-

      

When a star 1A.U.  away from the Earth produces a parallax of 1arcsec (1”), then it’s said to be 1 parsec away. 1 parsec = 3.2 ly

               

                 1 parsec = 3.2 ly

PARALLAX is only useful for the objects which are comparably closer to us, as the farther objects have negligible parallax angle they show a high error when we calculate their distances using parallax.

So, now for the objects which are far away from us, we use the “STANDARD CANDLE” method to calculate their distances.

Standard candles are the objects which has a known “LUMINOSITY”.

Luminosity is the total output power of an object and is measured in “watt (W)”.

There are 2 types of standard candles-

1.  CEPHEID VARIABLES
2.   TYPE 1A SUPERNOVA

Cepheid Variables-

Cepheid variables are the pulsating stars which show a change in their brightness at fixed intervals. The name Cepheid variables were given as they were first found in the constellation of Cephius.

Henrietta Swan Leavitt discovered these variable stars and also found out the method of finding their distances by her famous relations of Intensity vs. Time and Intensity vs. Time period.

  

              

This shows that the greater the pulsating period greater is the luminosity of the Star. So by using the relationship between luminosity and brightness we can find out its distance from us.

Brightness = Luminosity/[4π × (distance)²] 

It follows the inverse square law.

So, by using this relationship we can find the distance of the Star. We also do find the distances of the Galaxies far away from us. What we do is, we first find the Cepheid variables in that Galaxy and simply find it's distance. So hence, we also know the distance of that Galaxy.

But, this method is only useful up to 5Mpc to 50Mpc.

                           

                                                                               Cepheids as seen in night sky 

Type 1A Supernova-

Type 1A Supernova are formed due to the explosion of White Dwarfs that have a companion Star. Cause of presence of the companion Star, White Dwarfs pull all of its matter and get unstable and undergo gravitational collapse resulting in a Type 1A Supernova. They are so bright that they can outshine an entire Galaxy. Their absolute magnitude can reach up to -20.

They can easily be differentiated from other Supernova's as they lack the presence of hydrogen lines in its spectra and also they follow a distinctive curve as their brightness decreases.

Due to this, they are used as Standard Candles.

Type 1A Supernovae are used to measure distances from 1Mpc to 1000Mpc.

                A Type 1 Supernova Explosion

Magnitudes-

·      Magnitude of a star is the brightness of a star as measured from the Earth.

·      Hipparchus an ancient Greek astronomer invented the Magnitude scale.

·      He classified the stars according to their magnitudes. 

·      He classified them in 6 types in which "The first magnitude" were the brightest ones and "Sixth magnitude" were the faintest ones.

·      Immediately after Sunset the Stars which were visible were the bright Stars and were classified as "First Magnitude" or "Class 1" Stars. Later it was observed that some Stars which rise later on are even Brighter than the "First Magnitude" Stars. So, new scales were introduced where the magnitude was mentioned in negative values.

·      So, the from this scale we can say that "LOWER THE MAGNITUDE, BRIGHTER THE STAR IS".

·      As the magnitude of the Star Vega (Lyra) is close to 0, we consider it to the reference Star for magnitude scales.

We have 2 types of magnitude scales-

1. Apparent Magnitude-

The brightness of the Star measured from the Earth. The apparent magnitude of the Star depends upon the intrinsic luminosity, it's the distance from Earth, and any extinction of the light caused due to stellar dust.

           

As you can see in the diagram, the brightest star in the night sky Sirius (Cannis Major) has it's apparent magnitude -1. 46, the Sun has it's the apparent magnitude to be -26.72.

The human eye can only see till the Stars having apparent magnitude 6. Using a telescope, we can see even faintest stars as you can see, HST can observe stars having apparent magnitude 

Absolute Magnitude-

The absolute magnitude of a Star is measured in an inverse logarithmic astronomical magnitude scale.

For absolute magnitude, we consider all-stars to be at 10pc away from us. So, now all-stars are equidistant from us. Then, we measure their brightness.

                                 m - M = 5 log(d/10)

                                 where m = Apparent magnitude

                                 M = Absolute magnitude

                                 d = distance of the Star

               

As you can see in the table, the brightest Star in sky Sun having apparent magnitude -26.72 is now a faint star having an absolute magnitude of 4.2. This is because we are considering all-stars at 10pc (32.6 ly).

Using this method we can also tell whether the Star is closer to us that 10pc or far away from us than 10pc or is at 10pc. We simply compare the absolute magnitude and an apparent magnitude of the Star and if the absolute magnitude is greater than the apparent magnitude then the Star is at a distance greater than 10pc, if the absolute magnitude is lower than the apparent magnitude then the Star is at a distance lesser than 10pc if the absolute magnitude and the apparent magnitude are equal then the Star is at a distance of 10pc.