Distant Stars

Chapter 17

Solar Neighborhood

Milky Way has over 100

billion stars

We use parallax to find the

distance to stars

The parallax of one second

means that it is one parsec

away

Our Nearest NeighborsProxima Centauri in the Alpha Centauri

complex is 1.3 pc away (4.3 ly)

Barnard’s Star is 1.8 pc (6.0 ly)

Stellar MotionRadial Velocity – how fast it is moving toward or

away from us, this is measured using the

doppler shift

Transverse velocity – how fast perpendicular to

our line of sight, measured with careful

observation

Proper motion - The angular movement of a

star across the sky, as seen from Earth,

measured in seconds of arc per year. This

movement is a result of the star's actual motion

through space.

Will another star hit our system?

Alpha Centauri system

is slightly blue shifted

and moving toward us

but its transverse

velocity shows that it will

never get closer than

1 pc.

Brightness

Apparent brightness - The brightness that a star

appears to have, as measured by an observer

on Earth.

Absolute brightness - The apparent

brightness a star would have if it were

placed at a standard distance of 10 parsecs

from Earth.

In order to find the absolute brightness the

observer must find the apparent brightness

by viewing through a telescope and secondly

they must determine its distance through

parallax.

Magnitude ScaleA system of ranking stars by apparent brightness, developed by the Greek astronomer Hipparchus. Originally, the brightest stars in the sky were categorized as being of first magnitude, while the faintest stars visible to the naked eye were classified as sixth magnitude. The scheme has since been extended to cover stars and galaxies too faint to be seen by the unaided eye. Increasing magnitude means fainter stars, and a difference of 5 magnitudes corresponds to a factor of 100 in apparent brightness.

Temperature and ColorBy looking at stars in the

night sky you can see

which ones are hot and

cool

The colors of the cool red

star Betelgeuse and

the hot blue star Rigel

are clear

This has nothing to do

with Doppler shift

• Blackbody curves for three different

temperatures, along with the locations of the B

(blue) and V (visual) filters.

Star Classification• Between 1880 and 1920 the stellar spectra

were recorded for hundreds of stars

• They were organized on a scale of A to P with

A having the strongest hydrogen lines to P with

the weakest

• The scale was ordered to surface temperature

• The original letters now run O, B, A, F, G, K, M

• Use the word device Oh, Be A Fine Girl, Kiss

Me

• Astronomers further subdivide each

lettered spectral classification into 10

subdivisions, denoted by the numbers 0—

9. By convention, the lower the number,

the hotter the star. Thus, for example, our

Sun is classified as a G2 star (a little

cooler than G1 and a little hotter than G3),

Vega is a type A0, Barnard's Star is M5,

Betelgeuse is M2, and so on.

SPECTR

AL

CLA

SS

SURFACE

TEMPERA

TURE (K)

PROMINENT ABSORPTION LINESFAMILIAR

EXAMPLES

O 30,000 Ionized helium strong; multiply ionized

heavy elements; hydrogen faint

B 20,000 Neutral helium moderate; singly

ionized heavy elements; hydrogen

moderate

Rigel (B8)

A 10,000 Neutral helium very faint; singly ionized

heavy elements; hydrogen strong

Vega (A0), Sirius

(A1)

F 7,000 Singly ionized heavy elements; neutral

metals; hydrogen moderate

Canopus (F0)

G 6,000 Singly ionized heavy elements; neutral

metals; hydrogen relatively faint

Sun (G2), Alpha

Centauri (G2)

K 4,000 Singly ionized heavy elements; neutral

metals strong; hydrogen faint

Arcturus (K2),

Aldebaran

(K5)

M 3,000 Neutral atoms strong; molecules

moderate; hydrogen very faint

Betelgeuse (M2),

Barnard's Star

(M5)

Hertzsprung–Russell (H—R)

diagram• A plot of luminosity

against temperature (or spectral class) for a group of stars

Main Sequence

• Well-defined band on the Hertzsprung—Russell diagram, on which most stars are found, running from the top left of the diagram to the bottom right.

White

dwarf

region

white dwarf - A dwarf star with a surface

temperature that is hot, so that the object glows

white.

red giant - A giant star whose surface

temperature is relatively low, so that it glows with

a red color.

About 90 percent of all stars in our solar

neighborhood, and probably a similar

percentage elsewhere in the universe, are

main-sequence stars. About 9 percent of stars

are white dwarfs, and 1 percent are red giants.

Spectroscopic Parallax

• Method of determining the distance to a star by

measuring its temperature and then determining

its absolute brightness by comparing with a

standard H—R diagram. The absolute and

apparent brightnesses of the star give the star's

distance from Earth.

• A measurement of the apparent brightness of a

light source, combined with some knowledge of

its intrinsic properties, can yield an estimate of

the source's distance.

Luminosity Class• A classification scheme

which groups stars

according to the width of

their spectral lines. For a

group of stars with the

same temperature,

luminosity class

differentiates between

supergiants, giants, main-

sequence stars, and

subdwarfs.

TABLE 17.3

Stellar Luminosity Classes

CLASS DESCRIPTION

Ia Bright supergiants

Ib Supergiants

II Bright giants

III Giants

IV Subgiants

V Main-sequence stars/dwarfs

binary-star system

• A system which consists of two stars in

orbit about their common center of mass,

held together by their mutual gravitational

attraction. Most stars are found in binary-

star systems.

Can be visual binary, you

can see it, or spectroscopic

binary, using Doppler shift

• Binary properties can be determined indirectly

by measuring the periodic Doppler shift of one

star relative to the other as they move in their

orbits.

Mass of Stars • Mass, more than any other stellar property, determines a star's position on the main sequence. Stars that form with low mass will be cool and faint; they lie at the bottom of the main sequence. Very massive stars are hot and bright; they lie at the top of the main sequence.

The mass of a star can be

found using stellar data and

modifying Kepler’s Laws

- The relationship between

mass and radius along the

main sequence is that

they are proportional to

each other

- The relationship between

mass and luminosity is a

cubed relationship. If the

mass is doubled then the

luminosity is 8 times the

amount

Stellar Lifetime – The bigger they are,

the shorter they live

STAR SPECTRAL

TYPE

MASS (solar

masses)

CENTRAL

TEMPERATUR

E (106 K)

LUMINOSITY

(solar

luminosities)

ESTIMATED

LIFETIME (106

years)

Rigel B8Ia 10 30 44,000 20

Sirius A1V 2.3 20 23 1,000

Alpha

Centauri

G2V 1.1 17 1.4 7,000

Sun G2V 1.0 15 1.0 10,000

Proxima

Centauri

M5V 0.1 5.0 0.00006 >1,000,000

open cluster• Loosely bound

collection of tens to

hundreds of stars,

a few parsecs

across, generally

found in the plane

of the Milky Way.

• Pleiades

globular cluster• Tightly bound,

roughly spherical collection of hundreds of thousands, and sometimes millions, of stars, spanning about 50 parsecs. Globular clusters are distributed in the halos around the Milky Way and other galaxies.

• Omega Centauri