Photometry Filters

Photometry Filters

Color Index: Difference between two filtered magnitudes. For example, B-V.

Pe

rce

nta

ge

of

lig

ht

pa

ss

ing

th

rou

gh

fil

ter

(1

.0 =

10

0%

)

Star: An object that during some part of its lifetime derives 100% of its energy from the fusion of hydrogen nuclei (protons) to helium. Must have > 7% M_Sun.

Brown dwarf: An object intermediate in size between a planet and a star (~1 -> 7% M_Sun). May briefly fuse deuterium.

Planet: An object with a mass less than about 1% M_Sun. No fusion process occurs in them, and they shine

primarily by reflected light.

Apparent Brightness: How bright an object appears in the sky. A measure of the observed light received from a star or other object at the Earth.

Luminosity: The total energy radiated into space each second by a star or other object.

Standard Candle: An astronomical object of knownluminosity.

The Brightness of Stars

Star luminosities range from 0.0001 L_Sun up to 1,000,000 L_Sun.

Fig 16-7

Spectroscopy: The Key to the Universe

From detailed analysis of a star’s spectrum, one can determine:

• Radial velocity of star: Doppler shifting of spectral lines.• Stellar rotation rate: Doppler broadening of spectral lines.• Abundances of elements in star’s atmosphere:

Careful spectral analysis combined with knowledge of stellar temperatures and pressures. Nearly all stars ~75% H, ~25% He, + small amount of heavier elements

Table 17-1

OBAFGKM

40,000K -> 2,000K

Most stars ~75% H, ~25% He

€

106 →10−4 L_Sun

Up to ~few hundred km/s

Typically up to ~200 km/s

>100 M_Sun down to ~0.1 M_Sun

For “Normal” stars:

~25 R_Sun -> ~0.5 R_Sun

Fig 16-3

Measuring Brightness: The Magnitude Scale

Astronomical Photometry

A change of 5 magnitudes signifies a factor of 100 change in brightness.

Higher numbers = fainter object.

Faintest object visible to naked eye = +6 mag.

A change of 1 magnitude signifies a factor of ~2.5 change in brightness.

Characteristics of Stars on the H-R Diagram

• H-R Diagram: A plot of luminosity vs. surface temperature for stars.• Main Sequence: Location on the H-R diagram where the majority of stars (roughly 90%) fall. Stars here are actively fusing hydrogen into helium in their cores.• Giant, supergiant: Star with large luminosity but relatively low surface temperature, indicating a very large radius.• White Dwarf: Star with a mass roughly that of the sun, but with a volume only 1 millionth that of the sun (about the size of Earth). Can be hot but, due to extremely small size, are not very luminous. Very high density and surface gravity.

Fig 18-3, p.401

Parallax

Parallax: Defined to be one-half the angle that a a star shifts when seen from opposite sides of Earth’s orbit.

Units: arcseconds. There are 60 arcsec in 1 arcminute, and 60 arcminutes in a degree. Thus, there are 3600 arcsec in a degree. 360 degrees in a circle. (Full moon subtends 1/2 degree in sky.)

Determining Stellar Distances with Parallax

A parallax of 1 arcsec indicates a distance of 3.26 LY.

Define: 1 parsec (pc) = 3.26 LY.

€

D =1

pD = distance (in parsec)p = parallax (in arcsec)

Example: A star has a parallax of 0.1 arcsec. How far away is it in parsec? In LY?

Distances in the Universe• Earth to Moon: ~1 light second.• Earth to Sun: ~8 light minutes.• Sun to Pluto: ~5 light hours.• Sun to nearest star: ~4 light years (LY).• Sun to center of Milky Way Galaxy (MW) : ~30,000 LY.• Visible diameter of MW: ~100,000 LY.• MW to nearest ‘dwarf’ galaxy: ~75,000 LY.• MW to nearest major galaxy (Andromeda): ~2 million LY.• Distance to edge of visible universe: ~14 billion LY.

Measuring Distances in Astronomy• Parallax: Ground-based telescopes measure distances out to 60 LY. Space-based telescopes (Hipparchos) reach out to 300 LY.

Most accurate, and least controversial, technique.

• Cepheid Period-Luminosity Relation: Excellent extragalactic distance indicator. Provides accurate distances out to 100 million LY.

Two Other (less important) Techniques

• RR Lyrae stars as standard candles: Good out to ~2 Mly.• Spectroscopic parallax: In principle, can be used out to ~20 million LY for brightest stars, BUT not very accurate.