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TRANSCRIPT
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