ASTROPHYSICS 3Semester 1

Observational Astronomy&

Physics of Stars and Nebulae

Philip Best. pnb@roe.ac.uk

www.roe.ac.uk/~pnb/teaching.html

Key questions for this course

1) Understand the physical properties of stars. Stars show tight relations between their mass, luminosity, radius, etc. Where do these come from? We want to understand:

• The structure of stellar interiors• The energy generation process• The hydrostatic equilibrium• Radiative diffusion and convection• What controls the upper and lower mass limits of stars• The structure and properties of evolved compact stars

To do this, we will need to bring together aspects of many different fields of physics (nuclear, quantum, statistical, etc)

Key questions for this course

2) We want to understand how stars react with their environments

• What is the interstellar medium composed of, and why?• How do stars influence its properties?• What is the temperature and ionisation state of the gas?• What influence do stellar winds or supernovae have?• How does the interstellar medium affect our

observations of stars (dust extinction)?

3) Also in the course we will cover the basic principles of observational astronomy.

Course set-up

• Lectures Tuesday & Friday at 12.10, ROE Examples classes and discussion questions mixed in.

• Tutorials Monday, 11.10-13.00, Wks 3,5,7,9,11. JCMB rm 5326 Not assessed – but hand-in questions for feedback Hand in to me by the end of the previous Friday

• Revision class To be arranged, in April before the exam.

Radiative energy in the Universe

Parallax

Nearby stars move relative to background stars as the Earth orbits the sun.

Object with parallax of 1 arcsec has distance 1 parsec.

Johnson/Bessell filter transmissions

Black-body radiation: Planck Function

Spectra of different stellar spectral types

Spectra of different stellar spectral types

• Classification order:

O B A F G K M

Oh Be A Fine Kiss Me

Question: why is the strength of hydrogen absorption lines not monotonic with temp?

GirlGuy

Spectral properties of stars

• Question: why is the

Hotter stars have high ionisation species (He+, CIII, etc) and most of the hydrogen is fully ionised.

Cooler stars have low ionisation absorptions (metals, molecules).

Hertzsprung-Russell (HR) diagrams

Colour (B-V) vs absolute magnitude (Mv) for stars in the solar neighbourhood whose distances are reliably determined.

QuestionMost stars lie on the Main Sequence, but a small proportion are offset to the upper right in the colour ( Temperature) versus absolute magnitude ( luminosity) diagram. What does this tell us about these stars?

• Student A: These have similar properties to the other stars of the same luminosity (the A & B stars), the only difference being that

they are cooler, leading to the redder colours.

• Student B: These are like the other stars of the same temperature (the K & M stars), but are more luminous. Therefore they must be closer to the earth than the other stars.

• Student C: These are at the same temperature as the K & M stars, so to be more luminous they must be bigger.

• Student D: These cannot be black-body emitters

Which, if any, of these statements are correct. Why are the others wrong?

Hertzsprung-Russell (HR) diagrams

MK Classification:

I − Supergiants

II − Bright giants

III − Giants

IV − Subgiants

V − Main sequences (dwarfs)

VI − Sub dwarfs

Hertzsprung-Russell (HR) diagrams

Colour-colour diagram: reddening

The equatorial co-ordinate system

•Right Ascension: defined within equatorial plane measured from 0-24 hrs, increasing

eastwards 0h is direction of sun at vernal equinox

•Declination: measured perpendicular to RA due north is +90 deg, due south -90 deg.

•Precession: axes precess due to earth’s 26000yr

“wobble”, so co-ordinates need to be defined at a given epoch (e.g. J2000).

East and West…..

Declination values increase as you go up (north) in an image

N

Right Ascension increases to the East (left)

E

RA, Dec worked example

Two quasars are located at:

RA = 15h46m37.45s, Dec = +17°06´25.6"

and

RA = 15h46m58.13s, Dec = +17°03´47.1"

What is the angular separation of these two quasars on the sky (in arcsec)?

Discussion Question

You are observing at the UKIRT telescope in Hawaii

(latitude = +20, longitude = -155) in mid-January.

It is midnight.

Roughly what range of right ascensions and

declinations of stars are visible to you more than

30 degrees above the horizon?