Download File - greenscapes4you

All stars form in clouds
of dust and gas
Balance
of
pressure:
outward
from core
and
inward
from
gravity
We see different stars at different stages of their lives
Sirius is the brightest star in the
entire sky.
Only slightly larger than the , it is
only 8 light years away
SNAPSHOT OF THE HEAVENS
We see only one moment in any star’s life, and our
collective snapshot of the heavens consists of frozen
moments for billions of stars.
What do we now know about stars?
All stars form in a cloud of dust and gas called a
nebula.
They begin their life with roughly the same
composition.
Star’s mass at birth: about 75% hydrogen about
25% helium; less than 2% heavier elements
During most of star’s life, rate at which it generates
energy depends on balance between inward pull of
gravity and outward push of internal pressure from
nuclear fusion in core
Why are the stars we see different from each other?
Because we see different stars at various stages of
their lives
Early classification of stars took into account: the
star’s brightness and its location in sky
Greek letters: Alpha is brightest in constellation. Alpha
Leo, a.k.a. Regulus. (it’s on the ecliptic, the same
path that planets appear to take across the sky.
Why is a star bright?
Extremely powerful OR unusually close
Are stars that appear next to each other in the sky
necessarily neighbors?
They could lie at significant differences from the Earth
STELLAR LUMINOSITY
Luminosity of a star – the total amount of power
it radiates into space (expressed in watts)
The Sun’s luminosity is 3.8 x 10 (26) watts
A star’s luminosity cannot be measured directly.
Sun and Alpha Centauri A have the same
luminosity, but the Sun is much, much brighter.
Alpha Centauri A is about 270,000 times farther
from the earth than the Sun
Light obeys the inverse square law
If we viewed the Sun at twice the distance it would
appear 2(2) or 4 times dimmer.
Luminosities of stars are compared to the Sun’s
luminosity (L-Sun = 3.8 x 10 (26) watts)
Proxima Centauri is 0.0006 L-Sun;
Betelgeuse is 38,000 L-Sun
A star’s apparent brightness is measured with a
detector, such as a CCD, that records how much energy
strikes its light-sensitive surface each second.
Total luminosity and total apparent brightness take
into account all photons across the entire
electromagnetic spectrum.
Once a star’s apparent brightness has been measured,
the next step in determining it’s luminosity is to
measure its distance.
The most direct way to measure the distances to stars is with stellar
parallax. This is the small shift in a star’s apparent position caused by
the Earth’s motion around the Sun. Astronomers measure stellar
parallax by comparing observations of a nearby star made 6 months
apart. The nearby star appears to shift against the background of
more distant stars because we are observing it from 2 opposite points
of the earth’s orbit. Parallax can only be used to measure the
distances to stars within a few hundred light years away. (our “local”
solar neighborhood)
Proxima Centauri, the closest star to us, has a parallax angle of only
0.77 arc-second
The distance to an object with a parallax angle of 1 arc-second is
one parsec (pc)
1 pc = 3.26 light-years = 3.09 x 10(13) km
Enough stars have measurable parallax to give us a
fairly good example of the many different types of
stars. 300 stars within 33 light-years (10 parsecs) of
the Sun.
Angular distance in the sky:
Width of stretched out hand ~ 20 degrees
Width of fist ~ 10 degrees
Finger width ~ 1 degree
60 arcminutes per degree; 60 arc-seconds
per arc-minute
If the Sun were a distance of 10 parsecs from the Earth, it
would have an absolute magnitude of 4.8. It would be visible,
but not conspicuous on a dark night.
STELLAR SURFACE TEMPERATURE
Surface temperature is determined directly by the star’s
color or spectrum. A stars surface temperature
determines the color of light it emits.
The naked eye can distinguish colors only for the brightest
stars. Colors of stars become more evident when viewed
through binoculars or telescope.
Betelgeuse, a cool, red star would look much brighter
through a red filter than a blue filter.
Most important property of a star.
Most dependable method of
“weighing” a star is to use
Newton’s version of Kepler’s Third
Law - Universal law of Gravitation.
Stellar masses can only be
measured in binary star systems in
which the orbital properties of the
two stars have been determined.
Mizar, the second star in the handle of the Big Dipper
is actually two stars – a visual binary system
THE HERTZSPRUNG-RUSSELL DIAGRAM
Most stars fall along the main sequence – upper left to lower
right. These stars fuse hydrogen into helium in their cores and
have a wide range of life spans, which depend on their mass.
Higher mass stars on main sequence have shorter life spans.
A star has a limited supply of core hydrogen and therefore can
remain as a hydrogen-fusing main sequence star for a limited time
- the star’s main sequence lifetime.
Our Sun’s main sequence lifetime is about 10 billion years. A 30 x
M-Sun star has 30 times more H than the Sun, but burns it with a
luminosity that is 30,000 times greater. It’s lifetime is 30/30,000 =
1/10,000 as long as the Sun – corresponding to a lifetime of only a
few million years. This is a very short time, cosmically speaking.
This is one reason why massive stars are so rare. Most of the
massive stars that have ever been born are long since dead.
Betelgeuse
Orion
the
Hunter
Supergiants are very large in addition to being very
bright. Giants are somewhat smaller in radius and lower in
luminosity, but still much brighter than main sequence stars of
same spectral type. The hot, white, small radius stars near the
lower left are called white dwarfs.
Giants and Supergiants are stars nearing the ends of their lives
because they have already exhausted their core hydrogen.
Surprisingly, they grow more luminous when they begin to run
out of fuel. If a supergiant were in the same spot as the Sun, it
would engulf all of the planets through Jupiter!
STAR CLUSTERS
All stars are born from giant clouds of gas. Because a single
interstellar cloud can contain enough material to form many
stars, stars almost inevitably form in groups. Many stars still
congregate in the groups in which they formed.
The two basic types of groups: open clusters and globular
clusters.
Open clusters are always found in the disk of a galaxy and can
contain up to several thousand stars and typically span about 30
light years (10pc). The most famous open cluster is is the
Pleiades (also called “the seven sisters”) in the constellation
Taurus.
The Pleiades (to the right of Taurus the Bull in the sky)
is an open star cluster
Globular clusters are found in both the halo and disk of our
galaxy. They can contain more than a million stars,
concentrated in a ball typically 60-150 light years across. The
innermost region of a globular cluster can have 10,000 stars
packed within just a few light years!
M-13
Globular cluster in
Hercules
What would the view be like from a planet
in the midst of a globular cluster?
Star clusters are useful to astronomers
for two key reasons:
1. All the stars in a cluster lie within the same
distance from Earth
2. All the stars in a cluster formed at about the same
time (within a few million years of each other)