STARS MORE OF THEM

 

How did Astronomers initially calculate the distance of stars from the earth? One method is to use the parallax. If a person keeps a finger in front of eyes and closes one eye the object viewed looks at one place. If we close that eye and open another then the object looks to be at a different place. Based on the deviation of the object on account of parallax, the distance can be calculated.

However, the stars are so far away that our parallax method would not work even if we take opposite points on the earth to measure the parallax from stars. The Astronomers therefore measure the parallax of a star today and after 6 months so they can take benefit of the distance of the earth’s orbit around the sun to get sufficient parallax. This is about 30 crore Kilometres and therefore provides sufficient parallax to measure the distance of the nearby stars.

In 1837 as astronomer called Struve measured the distance of 61 Cygni which is about 11.4 light years away from us using this method. In 1838 Bessel measured the distance to Alpha Centauri by this method to be 4.3 light years. However, measuring the distance of far-off stars is not possible with this method because the parallax tends to be negligible for stars at greater distances even with Earth’s orbit as a base. With the Parallax method scientists cannot measure the distances of the stars over 400 light years. Therefore astronomers use other methods for measuring them.   

Another method for measuring the distance of stars employs stars known as variable stars. These are stars whose light varies with time. These stars actually constrict suddenly because of their instability, and because of that their temperature increases while the light emission decreases. Then again they expand and come back to their original size and luminescence. There are many types of variable stars but the important among them are the “Cepheid Variables”, Long Period Variables, and the “ R R Lyrie stars”.

The first variable star discovered by the Astronomers was Delta Cephei and on its name other similar stars are named Cepheid variables. There are 2 types of Cepheid variables 1. Classical Cepheids and 2. Type II Cepheids.

Classical Cepheids are Yellow Super Giant stars. There are about 500 such stars in our galaxy. They suddenly increase their luminosity by about 1 magnitude or to 2.5 times its actual brightness. Then, they gradually come back to their original luminosity in 1 to 5 days. Even without a telescope we can see about a dozen of such Classical Cepheids. The pole star or Polaris is a Classical Cepheid.  

Type II Cepheids take about 12 to 20 days to get back to their original luminosity.

The Long Period Variables take up to 1 year to come back to their original luminosity.

The RR Lyrie variables take only half a day to come back to their original brightness and some of them do so in only 1.5 hours. These stars are Blue Giants.

How are distances measured by using these variable stars?

Just outside our Galaxy is a small nebula called Smaller Magellanic Cloud. There are many variable stars in that nebula. Now all the stars of that nebula can be treated at the same distance from us despite them being light years apart from one another. In that nebula, some variable stars are less bright and some are brighter. That means as they are equidistant from us their comparative brightness is true and is not just apparent.

An American astronomer named Henrietta Leavitt by observation discovered that variable stars with higher brightness are returning back to normal slowly while stars of lower brightness are doing so rapidly. Thus she deduced that wherever such stars may be in the Universe they obey the same principle. This makes it possible for Astronomers to measure the distances of variable stars.

Supposing 2 different Cepheids are located in different directions in the sky. If they light up and come back to normal in the same duration, we can understand that both are of the same absolute luminosity. Now, if these stars appear to us with different luminosity on observation, then their relative distances to us can be estimated. Thereafter if we can measure the distance of one such variable star which is nearby through parallax, based on that the distances of the other Cepheids can be calculated.

While the parallax method enables us to estimate the distances of only thousands of stars, the variables method enables us to measure the distances of millions of stars.

However, a Spectrometer using the light from the star can reveal many properties of the star like its chemical composition, temperature, density, mass, distance, luminosity, and relative motion. Fraunhoper initiated optical spectroscopy in the early 1800’s when he passed light from Venus, Moon, Mars & some stars through a Prism and observed the lines.