The Magnitudes of Stars

Probably the first thing anyone notices when they glance up into the night sky is that the stars have different brightnesses. A small handful are bright, a few more ari.j fairly bright, but the majority art faisit. This characteristic - the brightness of a star - ts called the magnitude ol a star for any astronomical object that is observed using the naked eye). It is one of the oldest scientific classifications used today, and was invented by the Greek astronomer Hipparchus. He classified the...

K

A very bright Mira-type variable star and a favourite with amateur astronomers, like so many other red giants, its low temperature of 2000 K is in some doubt. It a deep red colour. This star is often cited as being a perfect introductory star for t o wish to observe a variable star. It is also an AGB star (see the section on Star Dea Star Map 19. Materai i.-l n oniony prawani aulaf k rti All the stars that have a mass greater than, or equal to, the Sun's will eventually become red giants. But...

Info

4.13 High-Mass Stars Nuclear Burning and an Onion We have seen what happens to those stars that have a low mass, and how they age and fade away gracefully. We now turn our attention to the high-mass stars. And as you have probably surmised by now, the death throes of these stars are very different from those of low-mass stars, and very spectacular. Throughout the entire life of a low-mass star, that is, one that is less than 4 M0, only two nuclear reactions occur - hydrogen burning and helium...

I

Where is the brightness of the star in W7m . is the star's luminosity in W and d is the distance to the star in metres. We can apply this to the Sun that is at a distance ol 1.50 x i0u m. This means that, say, a detector with an area of 1 square metre possibly a reflecting telescope) will receive 1370 watts of power from the Sun, Material .iironion , prawon auUirsi Phi To determine the luminosity of a star we need to know its distance and apparent brightness. We can achieve this quite easily by...

Co

The double cluster NGC 1 850, found in one of our neighbouring galaxies, the Large Magellanic Cloud, is an eye-catching object. It is a young, globular-like star cluster - a type of object unknown in our own Milky Way Galaxy. Moreover, NGC 1 850 is surrounded by a filigree pattern of diffuse gas, which scientists believe was created by the explosion of massive stars. NGC 1 850, imaged here with the NASA Hubble Space Telescope, is an unusual double cluster that lies in the bar of the...

Emission Nebulae

These clouds of gas are associated with very hot O- and EMype stars which produce immense amounts of ultraviolet radiation. They can have masses typically of about 100 to 10,000 solar masses. However, this huge mass is usually spread of a correspondingly large area 1 Recall that in fad the ISM is made up of sbout 74 hydrogen (by mass), 25 helium and the rest is metals. M a ten n f. h re i 'i i oi iy brawe m au i urs km possibly a tew light years across, so that the actual density of the gas is...

Obafgkm Lrns

The sequence goes from hot blue stars types O and A to cool red stars K and Af. and L In addition there are rare and hot stars called Wotf-Rayei stars, class WC and WNf exploding stars Q, and peculiar stars, P. The star types R> N and S, actually overlap class M, and so R and N have been reclassified as C-type stars, the C standing for Carbon stars. A new class has recently been introduced, the L class.29 Furthermore, the spectral types themselves are divided into ten spectral classes...

The End of an AGB Stars Life

As it ages, the AGB star continues to grow in size and increase its luminosity, along with an increase in the rate at which it loses mass. As mentioned earlier* the mass loss can be 104 Mq per year, which means that if the Sun lost mass at this rate, it would only last for 10,000 years. So, obviously, even giant stars cannot carry on this way for very long. If a star has a mass less that about 8 M , its stellar wind will soon strip away the outer layers almost down to the degenerate core....

Mass loss and Mass Gain

Having read the previous sections you may have gained the idea that star formation is simply a matter of material falling inward due to gravity. In fact, most of the material thai makes up a cloud is ejected into space and never forms any stars at all. This ejected material can help sweep away the gas and dust that's surrounding the young stars and make them visible to us. Several examples of such a process can be seen in the Rosette Nebula, the Trifid Nebula and the Bubble Nebula, mentioned...

Ngc 1501

A very nice blue planetary nebula, easily seen in telescopes of 20 cm, and glimpsed in apertures of 10 cm. With larger aperture, some structure can be glimpsed, and many observers liken this planetary nebula to that of the Eskimo nebula. See Star Map 71. Leo Minor . . Ursa Minof- Draco _ Hercu,e5. Hericher i. . Messier 27t _ * Aurjga, - > 76 Caktatolt 22 PK20640 1 Cefus A'quarius We now look at the final end point for low mass stars, and it is a very strange...

Vii

It's evident that astronomers use a complex and very confusing system In fact several classes of spectral type are no longer in use, and the luminosity classification is also open to confusion, It will not surprise you to know that there is even disagreement among astronomers as to whether, for example a star labelled Fy should be reclassified as GO Nevertheless, it is the system generally used, and so will be adhered to here, Examples of classification are I conclude my section on spectral...