1. Stellar position, distance, and properties

2. sTELLAR DISTANCE Units for measuring long distances:
Astronomical unit - distance btw Earth and Sun million km
Light-year - distance light travels in one year = 9.5 x1012 km
Parsec (pc) - parallax second - equal to 3.26 ly = x1013 km

3. STELLAR POSITION
Because Earth orbits around Sun, astronomers experience parallax when observing stars
At different times of year, nearby stars don’t seem to be in same position against backdrop of distant stars
Parallax - apparent shift in position caused by motion of observer

4. parallax
Calculate distance to star by knowing angle between two observed positions and distance between observation points
Closer star --> larger shift

5. Apparent Magnitude
Apparent magnitude - how bright a star appears to be to an observer on Earth
The lower the magnitude number, the brighter the star
Magnitude differs by a factor of 2.5

6. apparent magnitude
Ex: magnitude +1 star is 100 times brighter than magnitude +6 star
Does not indicate actual brightness of star because it doesn’t account for distance

7. luminosity Luminosity - actual brightness of a star
Luminosity depends on: size and temperature
Larger stars are more luminous
Hotter stars are more luminous
Astronomers use scale of absolute magnitude to express true luminosity
Sun’s luminosity = 3.85 x1026 W
Vary widely from to more than 1 million times Sun’s

8. absolute magnitude
Absolute magnitude - how bright a star would appear if all stars were at same distance from Earth - 10 pc
Allows comparisons of brightness at equal distances
Can only be calculated when actual distance to star is known

9. colour Range of colour star emits depends on temperature
Red < Orange < Yellow < White < Blue
Stars are grouped by temperature and colour into spectral classes (O, B, A, F, G, K, M)

10. Homework
Read pages 617 to 624 and do Q# 2-6