STARS AND GALAXIES
Part 3: Characteristics of Stars
1
Basic Characteristics
• Composition (Elements made of)
• Color and Temperature
• Brightness
• Size
• Distance from Earth
• Mass
Brightness and color can be used to
determine many of the other characteristics.
2
Spectra of Elements
• If you heat an
element until it
gives off light, the
light given off will
have a
characteristic
spectrum revealed
by a spectroscope.
• Video: The Temperature
and Composition of
Stars
Hydrogen
Helium
Carbon
3
Composition
• The spectrum of a star
tells us the composition
of a star (the elements it
is made of.)
• The black lines in the
spectrum are absorption
lines, and they can be
used to determine
composition of the star
by comparing their
location to the known
spectra of elements.
Spectrum of a star. Graph of spectral
luminosity versus wavelength.
4
Composition
• Most stars are composed predominately of
hydrogen, the lightest and most basic element in
the universe.
• Helium is the second most common element in a
typical star.
• Hydrogen and Helium = 96-99% of a star’s mass.
• Other elements often include oxygen, neon,
carbon, and nitrogen and even heavier elements
such as calcium and iron.
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Color and Temperature
• While our Sun is yellow, stars come in
many colors.
• Astronomers study a star's spectrum to
determine the dominant color of the star.
• Recall that the color of light is directly
related to its energy:
Higher Energy
Hotter
Lower Energy
Cooler
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Color and Temperature
• The color of a star
indicates its surface
temperature.
• Red stars are 3,000 K or
less, while blue stars can
be up to 60,000 K.
70000
Surface Temperature (K)
• The sun’s surface
temperature is about
5,800 K, while its core
temperature is as high as
15,000,000 K.
Temperature Ranges for Spectral
Classes of Stars
60000
50000
40000
30000
Low
20000
High
10000
0
Spectral Class
Review: K is Kelvin and 0°C = 273 K
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Brightness of Stars
Depends upon three
factors:
1. The temperature of
the star
2. The star’s size
3. The star’s distance
from Earth
• Apparent magnitude:
the brightness of a
star as it appears on
Earth
• Absolute magnitude:
the amount of light
actually given off by a
star; calculated by
astronomers
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Brightness of Stars – Variable Stars
• Brightness is constant for most stars.
• Some stars vary in brightness and are
called variable stars.
• A star may be variable because something
blocks light from reaching the Earth, as with
the binary star Algol, or because the light
being emitted varies.
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Brightness of Stars – Variable Stars
• A star that varies in both brightness and
size in a regular cycle is called a pulsating
variable star.
• The North Star (Polaris) is an example of a
Cepheid variable, one kind of pulsating
variable star. FYI: Polaris dims and
brightens in a 4-day cycle, but not enough
to detect with the naked eye.
10
Hertzsprung-Russell (H-R) Diagram
In the early 1900s,
Danish astronomer Ejnar
Hertzsprung and
American astronomer
Henry Norris Russell,
working independently,
found that as the
absolute magnitude of
stars increases, the
temperature usually also
increases.
Idealized graphic, not the plot of actual stars
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Hertzsprung-Russell (H-R) Diagram
• Remember color
indicates surface
temperature
• Plotting magnitude
vs. color reveals that
most stars fall within
several groups.
• Main sequence stars
(90%) are hydrogenburning stars
Idealized graphic, not the plot of actual stars
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Hertzsprung-Russell (H-R) Diagram
• (Red) Giants and
Supergiants –
generally much
larger than main
sequence stars
• These stars have
begun to die. They
have depleted their
hydrogen and now
burn heavier
elements.
Idealized graphic, not the plot of actual stars
Supergiants: Rigel (blue), Betelgeuse, Antares
Giant Star: Aldebaran
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Hertzsprung-Russell (H-R) Diagram
• White Dwarfs – as a red
giant continues to die it
eventually transforms
into a white dwarf
• A typical white dwarf is
about as massive as the
Sun, yet only slightly
bigger than the Earth.
• A white dwarf will
continue to lose energy
until it is a non-emitting
ball of gas, a black
dwarf.
Idealized graphic, not the plot of actual stars
Example: Sirius B, part of the Sirius Binary System, brightest star in the sky
14
HertzsprungRussell
Diagram
Diagram with 23,000
stars plotted.
From atlasoftheuniverse.com via
Wikimedia Commons
15
So How Large Are Stars?
• Stars vary tremendously
in size
• The Sun, with a diameter
of 1,392,000km, is a
medium size star.
• Stars from 1/10th to 10
times the size of the Sun
are considered mediumsized and this includes
most stars.
The Sun photographed by
NASA's Solar Dynamics
Observatory (SDO). This is a false
color image of the sun observed
in the extreme ultraviolet region
of the spectrum.
Scientific Notation: 1,392,000km = 1.392 X 106km or 1.392E6
16
17
The Smallest Star – A Neutron Star
• 16-20 km in diameter
• Mass of about 1.4 times
that of our Sun.
• A neutron star is
so dense that on Earth,
one teaspoonful would
weigh a 100 million tons!
• Like a white dwarf, a
neutron star is the
remains of a dying star,
but one that exploded in
a supernova.
The first direct observation of a
neutron star in visible light.
18
Measuring Star Distance
• Most commonly measured in light years.
(Remember: a light year is the distance light
can travel in one year. It is equal to 9.46 x
1012 km.)
• How we calculate the distance to a star
depends upon how far away the star is.
19
Parallax
TRY IT: If you hold your finger in front of your face
and close one eye and look with the other, then
switch eyes, you'll see your finger seem to "shift"
with respect to more distant objects behind it. This is
because your eyes are separated from each other by
a few inches - so each eye sees the finger in front of
you from a slightly different angle. The amount your
finger seems to shift is called its "parallax".
(http://heasarc.nasa.gov/docs/cosmic/nearest_star_info.html )
What happens to the amount
of shift as the object being
observed moves further away?
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Stellar Parallax
Note: The diagram to the
right is exaggerated for
teaching purposes.
• The distance the Earth
travels is very small
compared to the distance
to the star, so the parallax
angle (shift) is very, very
small, even for close stars.
• Using that angle, one can
calculate the distance to
the star.
Diagram from Windows to the Universe:
http://www.windows2universe.org/kids_
space/star_dist.html
21
Stellar Parallax
• Because the parallax angle gets increasingly small
the further away the star is, parallax can only be
used to calculate the distance to stars closer than
400 light-years away, and is most accurate for close
stars.
• FYI: Hipparcos was a satellite that operated between
1989 and 1993. Its purpose was the accurate
measurement of the positions of celestial
objects. This permitted the relatively accurate
determination of distance to stars up to 400 light
years away using parallax measurements (versus
100 light years using measurements made on Earth).
22
Beyond 400 Light Years
• For stars beyond 400 light years away,
astronomers use a comparison between
absolute magnitude and apparent
magnitude to estimate distance.
• What do you think is the biggest challenge
to doing this?
23
Why Stars Shine
• In the core of a star, the forces of gravity are
extremely strong.
• As matter is crunched together tighter and tighter by
gravity it heats up to millions of Kelvins.
• This ignites the process of nuclear
fusion, causing hydrogen atoms to
fuse together.
• As atoms are fused, some of the
matter is converted energy and given
off as electromagnetic waves.
http://www.swpc.noaa.gov/primer/primer.html
FYI: In our Sun, 4.2 billion kg of mass are
converted to energy each second.
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