ASTROPHYSICS 3 Semester 1 Observational Astronomy

1
ASTROPHYSICS 3Semester 1Observational
AstronomyPhysics of Stars and Nebulae

• Philip Best. pnb_at_roe.ac.uk
• www.roe.ac.uk/pnb/teaching.html

2
Key questions for this course

• 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)

3
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.

4
(No Transcript)
5
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.

6
Radiative energy in the Universe
7
Parallax

• Nearby stars move relative to background stars as
  the Earth orbits the sun.
• Object with parallax of 1 arcsec has distance 1
  parsec.

8
Johnson/Bessell filter transmissions
9
Black-body radiation Planck Function
10
(No Transcript)
11
Spectra of different stellar spectral types
12
Spectra of different stellar spectral types

• Classification order
• O B A F G K M
• Oh Be A Fine Girl Kiss Me
• Question why is the strength
• of hydrogen absorption lines
• not monotonic with temp?

13
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).
14
Hertzsprung-Russell (HR) diagrams

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

15
Question

• Most 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 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?

16
Hertzsprung-Russell (HR) diagrams
17
Hertzsprung-Russell (HR) diagrams

• MK Classification
• I - Supergiants
• II - Bright giants
• III - Giants
• IV - Subgiants
• V - Main sequences (dwarfs)
• VI - Sub dwarfs

18
Colour-colour diagram reddening
19
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 earths 26000yr wobble, so
  co-ordinates need to be defined at a given epoch
  (e.g. J2000).

20
East and West..
N
Declination values increase as you go up (north)
in an image
E
Right Ascension increases to the East (left)
21
RA, Dec worked example

• Two quasars are located at
• RA 15h46m37.45s, Dec 170625.6"
• and
• RA 15h46m58.13s, Dec 170347.1"
• What is the angular separation of these two
  quasars on the sky (in arcsec)?

22
Question on Right Ascension and
Declination(from Sept 2000 past exam paper)