Star Properties

Apparent Magnitude

System of Hipparchus Group of brightest stars 1m

Stars about ½ as bright as 1m 2m

Stars about ½ as bright as 2m 3m

•

•

•

Naked Eye Limit 6m

Apparent Magnitude

19th century photographers learn how eye responds to light (Pogson) Doubling the brightness is not perceived as a

doubling by the eye Eye response is logarithmic

Ratio of 100 in brightness corresponds to a Difference of five magnitudes m of 5 100X in light m of 1 2.512X in light

Some Apparent Magnitudes

Sun -26.8 Full Moon -12.6 Venus at brightest -4.4 Sirius -1.5 Naked Eye Limit 6.0 Faintest Objects +30.0

Hubble

Learning the Brightness

Is a star bright... Because it really is a bright star? Because it is close to the Earth?

Stellar brightness depends on Luminosity Distance

Measuring Distance

Stellar Parallax

June

January

Sun

Stellar Parallax

June

January

Sun

1 AU

Parallax

Measuring Parallax

1 AU

1 parsec

1 arcsec

Stellar Parallax

pd

1

When p is measured in arcsecand d is measured in parsecs

One parsec:

206,265 AU

3.26 light years

Stellar Parallax

Nearest star to Sun (largest parallax) Cen p = 0.7 arcsec

Limit of accurate parallax 200 pcs (angles of 0.005 arcsec) Hipparcos satellite (120,000 stars measured to

0.001 arcsec)

Absolute Magnitude

The magnitude a star would have at 10 parsecs from the Sun.

The apparent (m) and absolute (M) magnitudes of a star at 10 pcs are the same.

M, m, and d are related. Knowing two allows you to compute the third.

Putting the Pieces into Place

Ejnar Hertsprung1911

Henry Norris Russell1913

Luminosity Classes

I Supergiants

II Bright Giants

III Giants

IV Subgiants

V Dwarfs

Luminosity Class implies Size

Consider the Sun and Capella

The SunG2V M=5 Capella

G2III M=0

Luminosity Class implies Size

Equal sized pieces of each star are equally bright

Capella is 100X brighter (5 magnitudes) Capella must have 100X as much area Surface area radius2

Capella must be 10X larger than Sun.

Luminosity Class in the Spectrum

A3Supergiant

A3Giant

A3Dwarf

Sun G2V

Vega A1V

Betelgeuse M1I

Which of these stars is hottest?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Can’t compare

Which of these stars is brightest?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Can’t compare

Which of these stars is smallest?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Can’t compare

Which of these stars is most distant?

1. Sun G2V

2. Vega A1V

3. Betelgeuse M1I

4. Can’t compare

Spectroscopic Parallax

• Observe the spectrum and apparent magnitude of a star

• Classify the spectrum

• Plot it on the H-R Diagram

• Read off the M

• From m and M compute distance

Main Sequence

Color Index

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 500 1000

Wavelength (nm)

Re

lativ

e E

nerg

y

B V

12000 K

7000 K

*

*

* *

Color Index

Star Temperature mB mV .

1 12000 K 2.0 2.4

2 7000 K 3.0 3.1

Color Index = mB - mV = B-V

1 B-V = 2.0 - 2.4 = -0.4

2 B-V = 3.0 - 3.1 = -0.1

Spectroscopic Parallax

Can now get distances to any object whose spectrum can be measured. Limit 5000 pcs

Study Tools

Review 1 Review 2

The Advantage of Color Index

Measures temperature just like Spectral Type Much easier to obtain

requires two measurements of brightness spectral type requires getting the spectrum

Color-Magnitude Diagrams

M

Spectral Type

Standard H-R Diagram

mV

B-V

Color-Magnitude Diagram

Color-Magnitude Diagrams

Useful for star clusters Can substitute mV for MV since you know all the

stars are the same distance away. Star Clusters

Open (galactic) Globular

Structure of the Milky Way

Open Clusters

Irregular shape Few tens to few hundred stars In the plane of the galaxy Young stars

Open clusters

M16

M45

M37

Color-Magnitude DiagramM45

0

2

4

6

8

10

12

14

16

18

-0.5 0 0.5 1 1.5 2B-V

mV

Globular Clusters

Spherical in shape Hundreds of thousands of stars Halo distribution about galactic nucleus Old stars

Globular Clusters

M5 M3

SFA Observatory

Color-Magnitude DiagramM3