Introduction to Astrophysics

Lecture 9: Stellar classification and stellar physics

The Sun seen in X-rays

Observable properties

Brightness:

Measure by absolute magnitude, or by the total power output of the star.

Colour:

Measure by spectral class (OBAFGKM), by temperature, or by colour index.

Spectral class Colour Temperature (K)

O Violet > 28,000

B Blue 10,000 to 28,000

A Blue 7,500 to 10,000

F Blue - white 6,000 to 7,500

G White - yellow 5,000 to 6,000

K Orange - red 3,500 to 5000

M Red < 3,500

The Hertzsprung-Russell Diagram

Usually abbreviated to HR Diagram.

This is a plot of luminosity against colour for a selection of stars. Usually either

1) Use local stars whose distances are known, so we can get the absolute magnitude, or

2) Study a star cluster, where it should be a good approximation to take all the stars as being at equal distance.

Types of star

Stars are not scattered randomly throughout the HR diagram, but fall into classes. They are

The main sequence

Giants and supergiants

White dwarfs

The Main SequenceMost stars reside in a broad band stretching from the top left (hot and luminous) to the bottom right (cold and faint).

The Sun lies pretty close to the centre of this band.

The main sequence consists of stars which are burning hydrogen as nuclear fuel.

Terminology: a star is said to be `on the main sequence’ if it lies in this band.

Giants and supergiants

Their luminosity is high because they are very large, and so have a big surface area to radiate from. Typically they may have a radius one hundred times that of the Sun.

The most luminous are known as supergiants.

The giants and supergiants are stars which have exhausted their supply of hydrogen fuel and are trying to survive by burning heavier nuclei such as helium.

These lie in the upper right of the HR diagram, meaning that they are cool but luminous.

White Dwarfs

These lie in the lower left of the HR diagram, meaning that they are hot but faint.

There are probably very large numbers of these, but they are not easy to detect.

White dwarfs are remnants of stars which have completely exhausted their nuclear fuel and died. They have no new source of energy and are cooling into obscurity.

Understanding stars: binary systems

To fully appreciate the physics behind the HR diagram, we need more information about stars than their colour and luminosity. Ideally, we want to know something about their masses.

A crucial tool to let us do that is to use binary star systems: perhaps as many as 50% of stars do not exist in isolation but instead are part of a two-star system known as a binary.

Warm up: the mass of the Sun

Why am I so confident that the mass of the Sun is 2 x 1030 kg?

The mass of the Sun is what keeps the Earth in orbit. The required force is F = v2 Mearth / RGravity provides F = G Msun Mearth / R2

Setting these equal gives Msun = v2R / G. We know all these

quantities so we can substitute them in.

So, the dynamics of orbits allows us to work out the masses. Things are a bit more complicated if the objects have similar masses or the orbits are elliptical; use extensions of Kepler’s Third Law.

Types of binary star

Visual binaries.

Spectroscopic binaries.

Eclipsing binaries.

Interacting binaries.

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Visual binaries

These are stars far enough apart that they can be seen as separate stars.

Computer animations of circular and elliptical orbits.

Despite the impression given by the animations, the stars in visual binaries have to have a large separation in order to be detected as separate stars, and so the orbits tend to be very slow.

The numbers on these orbits are the years when a star reaches a given position!

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Spectroscopic binaries

Most binaries are too close to be distinguished as separate stars. However, their properties can still be studied by looking at the behaviour of their spectra.

Eclipsing binaries

If the orbital plane is aligned with the Earth, then the stars may regularly eclipse one another, leading to the blocking of light and hence more clues to the stars’ properties, such as their size.

Interacting binaries

Artist’s impressions by

Mark Garlick

Some binary stars are so close together that they interact. For example, if one is a large star such as a red giant, then the companion star may suck material from the surface of the giant.