Welcome to Astronomy 402/602

Transcription

1 Welcome to Astronomy 402/602 Introductions Syllabus Telescope proposal Coordinate Systems (Lecture) Coordinate System Exercise Light (Lecture) Telescopes (Lecture)

2 Syllabus Course goals Course expectations - diversity of material including telescope observing and use of computers Grading Course work: Exercises, telescope proposal, project paper, grad student project

3 Telescope Proposal Why we write a proposal What is contained within a telescope proposal: Abstract Scientific justification Technical justification Timeline: Sept 16: Proposal outline due Oct 21: Telescope proposal due Oct 23: Time allocation committee meeting

4 Altitude-Azimuth Coordinates Horizon coordinates change with the location of the observer and change as the Earth rotates Altitude W N S Azimuth E

5 Altitude Azimuth Question A circumpolar star is observed to have an altitude of at upper transit and an altitude of at lower transit. Assume that these are true altitudes after correction for refraction and instrumental errors. What is the latitude of the observer?

6 Equatorial Coordinates Projection of latitude and longitude on the sky Right ascension (RA) is equivalent to longitude Declination (DEC) is equivalent to latitude Right ascension of zero is where the ecliptic crosses the celestial equator and measured from West to East (Vernal Equinox) Coordinates are fixed for distant (non-moving) celestial objects

7 Equatorial Coordinate Questions 1. What is the right ascension at the winter solstice? Can you find it on one of the globes? 2. What is the maximum declination the sun reaches? When does the sun reach that declination? 3. In Fairfax, VA, Latitude , what declination is overhead?

8 Coordinate Precession The location at which the sun crosses the celestial equator (vernal equinox) precesses with respect to the distant stars over time RA (α) and DEC (δ), change slowly over time Therefore, we need to specify the epoch in which the coordinate system is given Usually we give coordinates in J tied to the position in the sky in the year 2000

9 Galactic Coordinates Based on the Milky Way (which is inclined ~63 to the celestial equator Longitude zero point is towards the center of the galaxy and increases in the same direction as right ascension Galactic latitude is measured away from the plane of the Galaxy

10 Galactic Coordinate Questions 1. At approximately what Galactic coordinate would you expect the density of stars in the sky to be highest? 2. Approximately what direction might you expect to see the highest density of globular clusters? Why?

11 Time Time is a way of keeping track of the movements of the sun and celestial objects in the sky Solar/Civil/Universal time Tracks the average time between one noon and the next Time zones are a human convenience Universal time(ut) ties all time on Earth to the time in Greenwich Julian date Number of days since noon UT on January 1, 4713 BC JD = (Y-2000) + N + L Y = year, N = day of year, L = leap days between 2001 and day of interest

12 Time Cont. Sidereal Time: Based on Earth s rotation with respect to the stars rather than the sun, it is the RA at the meridian (transit) Hour angle is the angle between the meridian and a source (goes E->W)

13 Angular separation of 2 sources Note that when we figure this out we have to take into account that RA converges at the poles cos d = sinδ 1 sinδ 2 + cosδ 1 cosδ 2 cos(α 1 -α 2 ) For small d: d = sqrt(cos 2 δ Δα 2 +Δδ 2 )

14 Exercise 1, Problem 1 In pairs please work on the first problem in your first exercise (due September 9)

15 Light and Radiation Light as a wave: wavelength, frequency, color, energy c= λν E=hν Frequency is the number of crests (or troughs) that pass a point every second. The speed of light, c, is constant, 3x10 8 meters/second

16 The Colors of Light Color is an expression of the wavelength/frequency/energy of light

17 How are temperature and color related in a dense object?

18 cool gas cloud, temp= 60K, emits mostly lowfrequency radio radiation. A dim, young star (red), the star's atmosphere, at 600K, radiates most in the infrared. The sun's surface, at 6000 K, is brightest at visible wavelengths. Cluster of very bright stars, at 60,000 K, these stars radiate strongly in the ultraviolet. Blackbody Radiation

19 Sagittarius star cloud

20 A. Star A gives off more red light and looks redder B. Star B gives off more red light and looks redder C. Star A gives off more blue light and looks redder D. Star B gives off less red and blue light, but looks redder

21 Magnitude vs Flux Magnitude is a logarithmic measure of brightness Flux is the linear measure of the amount of energy falling on a given area in a given time. You can subtract fluxes, but not magnitudes directly to get a brightness difference. m = -2.5log(f) + c The zero point for magnitude is the star Vega m 2 - m 1 = -2.5log(f 2 /f 1 ) A difference of 5 magnitudes is a factor of 100 in brightness Absolute magnitude (M) is the brightness of an object at 10 pc m-m = 5 log (d/10) = 5log(d) - 5 d in parsecs

22 Filters Filters allow you to look at one section of an objects spectrum at a time. There are several standard filter sets in astronomy.

23 Filters allow you to sample different parts of the blackbody curve You can imagine that each band in the rainbow is a different filter What is the sign of the violet-red color for star A versus star B?

24 Colors The color of an object is the magnitude difference between two different filters: B-V = m B - m V For a blackbody (most things in astronomy are close to blackbodies) then the difference in color tells you something about the temperature of the blackbody