Main Classes of Stars Astrophysics Lesson 10

Homework  None, you have exams next week! Good Luck!

Learning Objectives To recap:-  Problems observing stars. To know that:-  Stars can be classified according to their brightness (apparent or absolute).  Stars can be classified according to their temperature.  Analysis of absorption spectra gives clues as to the make up and surface temperature of stars.

Problems of Observing Stars  Wien’s law, Stefan’s law and inverse square law can be used to work out the properties of stars.  Requires careful measurements of the intensity (Wm -2 ) but there are problems  how many can you think of?

Problems Observing Stars 1)The atmosphere only lets certain wavelengths through – it is opaque to others. 2)Man-made light pollution makes dim objects hard to observe. 3)The turbulence in the atmosphere causes variation in refractive index – blurs image.

Problems Observing Stars 4)If the weather is cloudy, observation is impossible. 5)Dust from pollutants can interfere with images. 6)The sensitivity of detectors depends on the wavelength – CCDs are used (not perfect).

These problems can be solved:  By moving observatories to the top of high mountains;  Mounting a telescope in an aeroplane (although good tracking depends on the skill of the pilot and absence of turbulence that can make for a bumpy ride);  Putting the telescope in orbit.

Spectra How many types of spectra do you know? Observed by splitting light using a prism or a diffraction grating.

Spectra How many types of spectra do you know?  Continuous spectrum  contain all possible wavelengths emitted by hot objects e.g. filament bulb.  Emission spectrum  only specific wavelengths from excited electrons in hot gases e.g. mercury lamps.  Absorption spectrum  shining white light through a cool gas e.g. Fraunhofer lines.  Band spectrum  energy levels associated with the vibration of electrons in bonds.

Absorption spectrum  Photons passing through a cool gas excite electrons to higher energy levels which then fall back to lower energy levels and emit photons.  Because they are emitted in random directions, against a complete spectrum of colours the emissions of photons would appear black.  This is an absorption spectrum.

Spectra Compare the emission spectrum & absorption spectrum – the lines match up

Classification by Temperature  Astronomers look for lines in the absorption spectrum which arise from electron transitions in hydrogen atoms.  The electron drops from high levels to the second, n=2 energy level photons are emitted.  These are lines are found in the visible part of e-m spectrum and are known as Balmer lines.

Line Strength Depends on Temperature  To observe Balmer lines the electrons in the hydrogen atoms need to be in the n=2 state.  For high T. many electrons are performing Balmer transitions, so there are strong absorption lines.  At even higher T, the majority of electrons will reach the n=3 level, so there will be less Balmer transitions.  At low T electrons generally remain in the ground state. Energy level changes are rare.

Line Intensity Depends on Temp.

Notice that:  Temperature decreases from left to right;  For a given intensity, two temperatures are possible.  To overcome this, the spectra of other elements are analysed.  Peak intensities of different elements are found, and this can tie down the temperature. The idea is shown on the next graph:

Intensity of Lines

Main Classes Spectral ClassIntrinsic ColourTemperature (K)Prominent Absorption Lines OBlue25, ,000He+, He, H BBlue11, ,000He, H ABlue-White7, ,000H (strongest) ionised metals FWhite6, ,500Ionised metals GYellow-White5, ,000Ionised & neutral metals KOrange3, ,000Neutral metals MRed<3,500Neutral atoms, TiO Oh Be A Fine Girl, Kiss Me!

For Example  Therefore in the Sun, the spectral lines would be seen for iron and calcium, indicating a surface temperature of about 6000 K.  Very hot stars show spectral lines for light elements while cool stars will show up heavy elements, and spectra for molecules as well.

Sub-divisions  The classifications of stars according to spectra are also subdivided further with numbers (e.g A5) to allow the surface temperature to be determined within about 5 %.

Question  What would you not see when looking at the spectrum of the red giant Betelgeuse? What elements would you expect to see?

Answer  You would see little evidence of Balmer Lines.  You would see spectral lines for iron and calcium.  You would also see evidence for molecules of titanium dioxide.

Question The graph shows part of the visible spectrum for the star Vega: (note that it is an absorption spectrum so the intensity dips to a minimum at the emitted wavelengths.) The absorption lines are due to excited hydrogen atoms. The wavelength of each absorption is given in nm.

Question (a) Explain how Hydrogen atoms produce these absorption lines. (b) The diagram below shows the first six energy levels of a hydrogen atom. State which is the largest energy transition which produces an absorption line in the visible spectrum of Vega.

Energy Levels of Hydrogen Atoms

Questions (c) State the value of the wavelength corresponding to this transition. (d) What is the name given to the series which gives rise to the visible region of the hydrogen spectrum? (e) For which spectral classes are these lines the dominant feature? (AQA Past Question)

Answer (a) Electrons are excited when atoms collide. They rise to a higher energy level by a discrete amount. As they fall to lower energy levels they emit photons of energy corresponding to the energy difference between the energy levels. Absorption lines against a continuous spectrum are seen as the photons emitted at particular wavelengths are emitted in all directions, so would produce a dark line.

Answer (b) The largest transition would give a photon of the shortest wavelength. This would be from n = 6 to n = 2. (c) This is at 410 nm (d) The lines are called the Balmer series. (e) Classes A, B and F