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Transcript
Stars
… how I wonder what you are.
7
Goals
Tie together some topics from earlier in the semester
to learn about stars:
• How do we know how far away stars are?
• How do we know how bright they really are?
• What are they like?
– Temperature
– Radius
– Mass
• What categories can we place them in?
7
Distances
•
•
•
•
How do we perceive distances here on Earth?
How do we know A is closer than B?
Can we apply these to objects in space?
Can we apply these to objects beyond the solar
system?
• How do we know how far away the stars are?
7
Stellar Parallax
• One proof of a heliocentric
Universe is stellar parallax.
– Tycho Brahe saw no parallax of
stars.
– Copernicus thought stars must be
too far away.
• Nearest star: Proxima Centauri
Parallax angle = 0.76 arcsec
– Tycho’s precision = 1 arcmin
7
The Parsec
• Triangles:
tan q = opposite/adjacent
• For small angles: tan q = q
q = (1 AU)/Distance
Distance = (1 AU)/q
• What is the distance of an
object with q = 1 arcsec?
Distance = 206,265 AU
• Call this distance 1 parsec (pc)
• 1 pc = 206,265 AU = 3.3 lightyears
7
Distances
1
Distance (in parsecs) 
parallax (in arcsec)
•
•
•
•
1 parsec = distance with a parallax of 1 arcsecond.
1 lightyear = distance light travels in one year.
1 pc = 206,265 AU = 3.3 lightyears
Closest star: Proxima Centauri
q = 0.76 arcsec
Distance = 1.3 pc or 4.3 lightyears
7
How far is this?
The Sun
Hawaii
Alpha
Centauri
New York
7
Star light, star
bright
• In lecture 5 we talked
about stellar magnitudes.
• Sirius is magnitude –1.5
Polaris is magnitude 2.5
• While Sirius is brighter
than Polaris, Sirius is
also a lot closer to us.
7
Apparent and Absolute
• Apparent Magnitude = the brightness (magnitude) of a
star as seen from the Earth.  m
– Depends on star’s total energy radiated (Luminosity) and its
distance
• Absolute Magnitude = the brightness (magnitude) of a
star at a distance of 10 pc.  M
– Only depends on a star’s luminosity
 distance
m  M  5log 10 
 10pc



7
example
 distance
m  M  5log 10 
 10pc



• Our Sun:
– m = -26.8,
– distance = 1/206,265 = 4.8 x 10-6 pc
So: M = 4.8
• Polaris:
– m = 2.5,
– distance = 132 pc
So: M = -3.1
• Polaris is 1500 times more luminous than the Sun!
7
Stellar Temperatures
Stellar Spectra
How hot are stars?
• In Lecture 3 we
learned about
blackbody
spectra and
temperature.
• Since different
stars have
different colors,
different stars
must be different
temperatures.
Hot
Cool
7
Spectral
Classifications
7
Orion Copyright – Tyler Nordgren
7
Stellar Masses
How massive are stars?
• Kepler’s Laws – devised for the planets.
• Apply to any object that orbits another object.
• Kepler’s Third Law relates:
– Period: “how long it takes to orbit something”
– Semimajor axis: “how far you are away from that something”
– Mass: “how much gravity is pulling you around in orbit”
3
a
P 
M
2
• Where M is the Total Mass.
• Can calculate the mass of stars this way.
7
Binary Stars
• Most stars in the
sky are in
multiple systems.
• Binaries, triplets,
quadruplets,
etc….
– Sirius
– Alcor and Mizar
• The Sun is in the
minority by being
single.
7
Spectroscopic Binaries
• Sometimes you can’t see both stars.
– Too close
– Too far
– Too faint
• But if a star is orbiting something it must be moving.
7
NPOI Observations of Mizar A
(1 Ursa Majoris)
0.01 arcsec
Orbital Phase: 000o
Mizar, 88 light years distant, is the middle star in the handle of
the Big Dipper. It was the first binary star system to be imaged
with a telescope. Spectroscopic observations show periodic
Doppler shifts in the spectra of Mizar A and B, indicating that they
are each binary stars. But they were too close to be directly
imaged - until 2 May 1996, when the NPOI produced the first
image of Mizar A. That image was the highest angular resolution
image ever made in optical astronomy. Since then, the NPOI has
observed Mizar A in 23 different positions over half the binary
orbit. These images have been combined here to make a movie
of the orbit. As a reference point, one component has been fixed
at the map center; in reality, the two stars are of comparable size
and revolve about a common central position.
7
Stellar Masses
How massive are stars?
3
a
P 
M
2
• Where M is the Total Mass of the binary.
• Most stars have masses calculated this way.
• Result:
– The more massive the star, the more luminous it is.
– The more massive the star, the hotter it is.
7
Stellar Radii
How big are stars?
• We see stars have different
luminosities and different
temperatures.
• Stars have different sizes.
• If you know:
50 mas
– Distance
– Angular size
• Learn real size.
7
Stars are small
• Betelgeuse is the only star big enough to directly
see its surface with a normal telescope.
7
Interferometry
• Combine the light from two or more telescopes to
simulate the RESOLUTION of one giant
telescope.
NPOI - optical
VLA - radio
7
Optical
Interferometry
• NPOI simulates a single optical
telescope 65 meters in diameter.
• Resolve stars as small as 1.5 mas!
PTI - infrared
7
Angular versus Linear
Supergiants, Giants and Dwarfs
7
H-R Diagram
• Can order the stars we see by the property of
temperature and luminosity (or absolute
magnitude).
Prominent stars
Nearby Stars
Brightest Stars
1000 pc Stars
7