Variable Stars

There are also stars the brightness of which varies over a period of time ranging from hours, days or weeks. Such stars are known as variable stars. Many of them make interesting objects for observation. There are basically two kinds of variable stars and their brightness varies due to completely different reasons. One class of variable stars known as Cepheid variables (see page 24) change in brightness due to periodic changes in the size of the star which pulsates regularly every few days or weeks. During this pulsation the radius of the star may change by as much as a few million kilometres. When a Cepheid has contracted to its limit, that is, it has reached its smallest size, the surface temperature is highest and the star reaches its peak brightness. At the other extreme, when the star has expanded to its largest dimensions, its surface cools and it attains its least brightness. Cepheid variables are important because astronomers use them as a standard for measuring distances of stars.

The other type of variable stars are the eclipsing binaries. Such a star is usually made up of two stars close by, one orbiting the other. If one of them is less bright than the other, then, every time the dimmer companion comes in front of the brighter star as seen from Earth, the overall brightness of the two appears to dim. Since the dimming is caused by eclipse of the brighter star by its less bright companion, it is called eclipsing variable (see also page 46).

Another thing we will notice about the stars is that they come in a wide range of colours. Some are bluish-white, some are yellow and still others have a deep orange colour. The colour of a star tells us something about its surface temperature. For example, as the electric element in a toaster or oven heats, the colour changes from a deep red to orange. If the temperature is increased further, it would turn yellow, then white and finally blue (by which time it would have melted). The sequence of the colours in the rainbow is matched in the metal by increasing the heat.

We see the same phenomenon in stars. The reddish-orange stars are the coolest (roughly 3000°C) while the bluish-white stars are the hottest (about 20/000°C). The yellow stars lie somewhere in-between.

The Changing Pattern

If we observe the night sky for a few weeks we will notice that it is constantly changing. The positions of the various constellations and stars as we see them tonight at a particular hour will not be the same at the same time tomorrow. This happens because on any night, at a particular place, the stars and constellations 'rise' about 4 minutes earlier than on the previous night. So the night sky at 9 p.m. on January 1 will be the same as at 8.56 p.m. on January 2, or at 8.52 p.m. on January 3, and so on. On January 16 the same star positions will be seen at 8.00 p.m. and on January 31 at 7.00 p.m. Thus we see that the constellation pattern moves constantly westward. At the end of the year, that is, after 12 months, the sky pattern is again the same as at the beginning.

This apparent westward motion of the constellations is caused by Earth's orbital motion around the Sun. As the Earth goes round the Sun, the stars on the dark (night) side become visible and those on the day side are blotted out by the Sun. The pattern changes progressively the year round.

An implication of all this is that on any particular date, at a particular hour, the position of the constellations in the night sky as seen from a particular place would be the same irrespective of the year. In other words, the pattern of stars in the night sky that we see at 10.00 p.m. on June 1, 1997, for example, is the same as it was on the night of June 1, 1977 and will be on June 1, 2017 at the same time. Only the positions of the planets would change for reasons we shall see later. Of course, the sky pattern does change, but it does so over a period of several thousand years due to the slow wobbling of Earth's axis as it spins like a top.

Timings for Using the same Sky Map on Different Dates

Jan. 1

9.00

p

.m.

Jan.

16

8.00

p.m.

Feb. 1

7.00

p

.m.

Feb.

16

6.00

p.m.

Mar. 1

5.00

p

.m.

Mar.

16

4.00

p.m.

Apr. 1

3.00

p

.m.

Apr.

16

2.00

p.m.

May. 1

1.00

p

.m.

May.

16

12.00

noon

Jun. 1

11.00

p

.m.

Jun.

16

10.00

a.m.

Jul. 1

9.00

a

m.

Jul.

16

8.00

a.m.

Aug. 1

7.00

a

m.

Aug.

16

6.00

a.m.

Sep. 1

5.00

a

.m.

Sep.

16

4.00

a.m.

Oct. 1

3.00

a

.m.

Oct.

16

2.00

a.m.

Nov. 1

1.00

a

.m.

Nov.

16

12.00

midnight

Dec. 1

11.00

p

.m.

Dec.

16

10.00

p.m.

But we cannot notice it during our lifetime.

Once we get familiar with the constellations we will be able to use the sky maps published in newspapers and magazines. The map for any month can be used for any other month but at different times and, of course, only during the dark hours. This is specially useful if we want to observe the constellations in the early hours of the morning when visibility is often the best. For example, the sky map shown in the beginning of this chapter can be used on October 1 at 3.00 a.m., on September 16 at 4.00 a.m., on September 1 at 5.00 a.m., or on August 16 at 6.00 a.m. We can work out the early morning timings for other sky maps in a similar way.

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