The radiation from stars is not a perfectly continuous spectrum-- some wavelengths are missing. The missing wavelengths correspond to a number of elements. The absorption takes place in the outer layers of the star therefore this means that we have a way of telling which elements are in the star (or at least in its outer layers).
A star that is moving relative to Earth will show a Doppler shift in spectrum.
Red shift: light from stars that are receding
Blue shift: light from stars which are approaching.
E.2.8 Describe the overall classification system of spectral classes.
Different stars give out different spectra of light which allows us to classify stars by their spectral class. Stars that emit the same type of spectrum are allocated to the same spectral class.
There are several main spectral classes in order of decreasing surface temperature.
O: Oh
B: Be
A: A
F: Fine
G: Guy
K: Kiss
M: Me
E.2.9 Describe the different types of star.
BINARY STARS:
- Some stars like our Sun exist by themselves, but many have a partner
- Binary stars rotate around their own common centre of mass
- By analysing the orbital period and separation, the mass of each star in the binary system can be found
RED GIANT STARS:
- Large in size
- Red in colour
- Comparatively cool
- Later possible stages for a star
- Source of energy: Fusion of some elements other than hydrogen
- Red supergiants are even larger
WHITE DWARF STARS:
- Small in size
- White in colour
- White therefore comparatively hot
- One of the final stages for some stars
- Fusion is no longer taking place - white dwarf is just a hot remnant that is cooling down
- Eventually it will cease to give out light when it becomes cold enough
- After ceasing to give out light (cold) it is called a brown dwarf
CEPHEID VARIABLES:
- They are stars that are a little unstable.
- Observed to have a regular variation in brightness and (therefore) luminosity
- Aforementioned variation due to oscillation in the size of the star
- Rare but useful because there is a link between period of brightness variation and average luminosity
- Astronomers can therefore use them to help calculate the distances to some galaxies
VISUAL BINARIES
Binary stars (e.g. Sirius A) that can be seen with the naked eye of with a telescope are called visual binaries, when they are further away from us or closer together, resolution become difficult.
SPECTROSCOPIC BINARY STARS
In some cases, stellar spectra can be used to deduce the presence of two stars - these are called spectroscopic binary stars. As stars move around their common centre of mass, one star will be approaching whilst the other is receding.
The diagram above shows a spectroscopic binary system. In the right hand diagram, Star A approaches and Star B recedes from our line of sight. Therefore the absorption lines of A are blue shifted and the absorption lines of B are red shifted (moving away so longer wavelength, therefore red). In the left hand diagram, because the motion of the stars relative to our line of sight is opposite, the shift is reversed.
ECLIPSING BINARY STARS
Show a periodic variation in the brightness of light emitted from the star system. This occurs because during their rotation, one star periodically obscures, or eclipses, the other.
The diagram above shows an eclipsing binary system.
Position-
1 and 3: light is reaching us at a maximum, because it is arriving directly from both stars
2 and 4: reduction in brightness as the stars are eclipsing each other.
E.2.11 Identify the general regions of star types on a Hertzsprung-Russell (HR) diagram.
Discovery by Hertzsprung and Russell in 1910: For most stars, there is a relationship between surface temperature and luminosity.
Dots on diagram below represent stars, scales are not linear.
Temperature scale: Runs backwards, high temperatures on the left.
Absolute magnitude: The apparent magnitude it would have if it were observed from a distance of 10 parsecs. Absolute magnitudes are much more negative than the apparent magnitudes of the stars.
- 90% of stars fall into the diagonal band known as the main sequence. It can be shown using the Stefan-Boltzmann Law that stars increase in size as we move up the main sequence.
- Lower right: The coolest stars, reddish in colour.
- In the middle: (further towards the left than lower right) hotter, more luminous stars that are yellow and white.
- Lower left: more luminous blue stars.
Mass of star increases moving up the main sequence so the gravitational pressure increases with mass. Therefore, to maintain equilibrium, fusion reactions in the core must generate a greater radiation pressure. The star has to burn at a higher temperature, giving it a greater luminosity.
9% of stars = red giants and supergiants. From Stefan-Boltzmann Law we can see that their high luminosity and low temperature means that they must have a very large area - they are therefore giants.
WHITE DWARFS are very hot but not luminous, therefore they are much smaller than their counterparts on the main sequence.
The cepheids, congregate in a great band of instability that appears between the main sequence and the red giants.
Images and diagrams from: Heinemenn HL Physics Course Companion, IB Physics study guide by Tim Kirk
9% of stars = red giants and supergiants. From Stefan-Boltzmann Law we can see that their high luminosity and low temperature means that they must have a very large area - they are therefore giants.
WHITE DWARFS are very hot but not luminous, therefore they are much smaller than their counterparts on the main sequence.
The cepheids, congregate in a great band of instability that appears between the main sequence and the red giants.
Images and diagrams from: Heinemenn HL Physics Course Companion, IB Physics study guide by Tim Kirk
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