Stellar position, distance, and properties

sTELLAR DISTANCE Units for measuring long distances: Astronomical unit - distance btw Earth and Sun - 150 million km Light-year - distance light travels in one year = 9.5 x1012 km Parsec (pc) - parallax second - equal to 3.26 ly = 3.086 x1013 km

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

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

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

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

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 0.0001 to more than 1 million times Sun’s

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

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)

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