PHY2083 ASTRONOMY Dr. Rubina Kotak r.kotak@qub.ac.uk Office F016 Dr. Chris Watson c.a.watson@qub.ac.uk Office S036
PHY2083 ASTRONOMY Weeks 1-6: Monday 10:00 DBB 0G.005 Wednesday 9:00 PFC 02/018 Friday 10:00 DBB 0G.005
Preliminaries: Module relies on use of previous maths, physics knowledge! Units: astronomy units e.g. distances in AU or pc not metres; solar masses, luminosities pdf files of lecture notes (summary) will be made available on QoL before the lectures in as far as possible. You should bring these to the lectures and annotate as required. The pdf files will NOT necessarily contain all the material discussed in the lectures => attend the lectures and supplement the notes with textbooks! Bring your calculator to the lectures! Feedback welcome! Have fun! Ask questions!
Assignments You will be given 2 assignments to complete over the course of the next 6 weeks. These will count for 10% of your module mark (also, good practice for exams!)
Plan for today Scales in the Universe Useful units (AU, light-year etc.) Basic concepts: parallax; radial velocities
Units of distance Why can t we simply use metres / kilometres / miles?
Astronomical Unit (AU) = mean Earth-Sun distance = 149 597 870 700 m ~ 1.49 x 10 11 m 1 AU AUs are useful for distances within the solar system e.g. Mercury ~ 0.39 AU; Neptune ~ 30 AU (from Sun)
A few basic concepts: Maxwell Einstein The speed of light is finite and constant ~ 300 000 km/s
The speed of light is finite and constant But for everyday purposes, light appears to travel instantaneously 1m
Light-year (ly) Unit of distance, not time! 1 ly = distance travelled by light in 1 year: 1 year = 365.25 d = 24 x 60 x 60 x 365.25 s = 31557600 s speed of light = 2.99 x 10 8 m/s (distance = speed x time) => 1 ly = 2.99 x 10 8 x 31557600 = 9.43 x 10 15 m ~ 63241 AU
It takes about 8 minutes for light from the Sun to reach us
Saturn: 1.4 billion km => 1.3 light-hours NASA
The speed of light is finite and constant => looking far out in space = looking back in time Light from Alpha Cen takes 4 years to reach us.
Virgo cluster of galaxies ~ 53 million light years away
The most distant object in the Universe is in this image! light from it has travelled 13.2 billion yrs
Examples of distances in ly: Nearest star to the solar system: Proxima Centauri ~ 4.2 ly Sirius: brightest star in the night sky ~ 8.6 ly Our galaxy (Milky Way) is about 1000 ly across The Andromeda galaxy (sister galaxy to Milky Way) ~ 2.5 x 10 6 ly Need a more manageable unit for distances to other galaxies
Parsec (pc) (see later for definition of parsec) 1 pc = 206265 AU ~ 3.26 ly Distances to galaxies, clusters of galaxies etc. given typically in kilo-parsecs (kpc) i.e., 10 3 pc, and mega-parsecs (Mpc) i.e., 10 6 pc
8200 pc Solar System
Velocities of celestial bodies Earth s Orbital Velocity v=29.8 km/sec
220 km/sec
A binary stellar system:
Angular measures I (used for distances, velocities) Size and scale are often specified by measuring lengths and angles. A full circle contains 360 degrees = 2π radians Each 1 degree increment can be sub-divided into arc minutes i.e., 60 arc minutes (60 ) = 1 degree Each arc minute can be divided into arc seconds i.e., 60 arc seconds (60 ) = 1 arc minute
Consider the velocity of a star relative to an observer V θ C v s B Vs can be decomposed into 2 mutually perpendicular components A v r D V r = AD = radial velocity; component along line-of-sight V θ = AC = transverse / tangential velocity; projected angular motion V s = AB = space velocity; motion of star relative to the sun
Proper Motion The angular distance a star appears to move in a year (after correction for the motion of the Earth) is its PROPER MOTION, µ. Measured in arc seconds per year. Most stars are too distant to have an appreciable µ. Largest value is for Barnard s star at 10.3 / yr
Barnard s star
Parallax Apparent change in direction of a star due to the orbital motion of the Earth around the Sun. On Earth, the distance to the peak of a distant mountain can be determined by measuring the angular position of the peak from 2 observation points separated by a known baseline distance. Distance to the peak follows from simple trigonometry. Triangulation
Triangulation d = B / (tan p) Of course, finding the distance to a star requires a baseline longer than the Earth s diameter. As the Earth orbits the sun, baseline = diameter of Earth s orbit around the Sun
Angular measures II (used for distances, velocities) 1 radian ~ 57.3 degrees ~ 3438 (arc mins) ~ 206265 (arc secs) 1 pc = 206265 AU ~ 3.26 ly A parsec is the distance at which the parallax of an object = 1 arcsecond distance (pc) = 1 / parallax (arcsecs)