Download 1. Stellar Evolution – Notes Astronomers classify stars according to

Stellar Evolution – Notes
Astronomers classify stars according to their physical characteristics. Characteristics used to classify
stars include composition, color, temperature, brightness, size, etc.
White light, such as sunlight, is actually a
combination of all the colors that appear in its
spectrum. When a beam of sunlight passes through a
glass prism, the light is broken into a rainbowcolored band called a spectrum. Because of its
electric and magnetic properties, light is also called
electromagnetic radiation. It has wavelike properties described by its wavelength. The full array of all
types of electromagnetic radiation is called the electromagnetic spectrum. It extends from the longestwavelength radio waves to the shortest-wavelength gamma rays. Visible light occupies only a tiny
portion of the full electromagnetic spectrum.
To learn about objects in the heavens, astronomers study the character of
the electromagnetic radiation coming from those objects. The light from
nearby planets, distant stars, and remote galaxies has characteristic
“fingerprints” that reveal the chemical composition of these celestial
objects. Stars vary in their chemical composition. Astronomers use
spectrographs to determine the elements found in stars. A spectrograph is
a device that breaks light into colors and produces an image of the
resulting spectrum.
A hot, dense object emits a continuous spectrum—a complete
rainbow of colors without any spectral lines.
A hot, transparent gas produces an emission line spectrum—a series
of bright spectral lines against a dark background. Each chemical
element produces its own unique pattern of spectral lines.
A cool, transparent gas in front of a source of a continuous spectrum
produces an absorption line spectrum—a series of dark spectral lines among the colors of the
continuous spectrum. Furthermore, the dark lines in the absorption spectrum of a particular gas occur
at exactly the same wavelengths as the bright lines in the emission spectrum of that same gas.
The light that moves outward through the sun is a continuous spectrum since the
interior regions of the sun have high density. However, when the light reaches
the low density region of the solar atmosphere, some colors of light are
absorbed. Thus, when astronomers take spectra of the sun and other stars they
see an absorption spectrum due to the absorption of solar atmosphere.
Astronomers group similar-appearing stellar spectra into spectral classes. Stars of different spectral
classes have spectra dominated by different absorption lines.
Astronomers have devised a classification scheme which describes the absorption lines of a spectrum.
They have seven spectral classes for stars (OBAFGKM). A traditional mnemonic for the sequence is
Oh, Be, A Fine Girl/Guy, Kiss Me!
Although based on the absorption lines, spectral class also tells you about the surface temperature of
the star. One can see few spectral lines in the early spectral classes O and B. This reflects the
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simplicity of atomic structure associated with high temperature. The later spectral classes K and M
have a large number of lines indicating the larger number of atomic structures possible at lower
temperatures.
Most brown dwarfs are in even cooler spectral classes
called L and T. Unlike true stars, brown dwarfs are too
small to sustain thermonuclear fusion.
The intensity of light from a relatively cool star peaks at
long wavelengths, making the star look red. A hot star’s
intensity curve peaks at shorter wavelengths so the star
looks blue. Red stars are relatively cold, with low
surface temperatures; blue stars are relatively hot, with
high surface temperatures.
The table below lists spectral class, peak wavelength, and color with the corresponding temperature.
Note that these are all different ways of talking about the surface temperature of a star.
spectral class
peak wavelength
color
temperature
O
72.5 nm
Blue
40,000 K
B
145 nm
Light Blue
20,000 K
A
290 nm
White
10,000 K
F
387 nm
Yellow-White
7,500 K
G
527 nm
Yellow
5,500 K
K
725 nm
Orange
4,000 K
M
966 nm
Red
3,000 K
The brightness of a star depends upon both its size and its temperature. How bright a star looks from
Earth depends on both its distance from Earth and how bright the star actually is. The brightness of a
star can be described in two different ways: apparent brightness and absolute brightness. A star’s
apparent brightness is its brightness as seen from Earth. Astronomers can measure apparent brightness
fairly easily using electronic devices. A star’s absolute brightness is the brightness the star would have
if it were at a standard distance from Earth.
Some of the tools used by modern astronomers are actually many centuries old. One such tool is the
magnitude scale to denote the brightness of stars. This scale was introduced in the second century B.C.
by the Greek astronomer Hipparchus, who called the brightest stars first-magnitude stars. Stars about
half as bright as first-magnitude stars were called second-magnitude stars, and so forth, down to sixthmagnitude stars, the dimmest ones he could see. After telescopes came into use, astronomers extended
Hipparchus’ magnitude scale to include even dimmer stars. To make computations easier, the
magnitude scale was redefined so that a magnitude difference of 5 corresponds exactly to a factor of
100 in brightness. A magnitude difference of 1 corresponds to a factor of approximately 2.512 in
brightness.
Astronomers denote the apparent brightness of objects in the sky by their apparent magnitudes. The
higher the apparent magnitude, the dimmer the object.
Absolute magnitude measures a star’s true energy output—its luminosity. Absolute magnitude is the
apparent magnitude a star would have if it were located exactly 10 parsecs from Earth.
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Luminosity is the total energy that a star produces in one second. It depends on both the radius of the
star and on its surface temperature. Thus, the luminosity of a star would increase if one increased either
the size R or the surface temperature T with temperature being the dominating factor.
Two important characteristics of stars are temperature and
luminosity. Ejnar Hertzsprung and Henry Norris-Russell
made a graph to find out whether these characteristics are
related. The graph they made is called the HertzsprungRussell diagram, or H-R diagram. Astronomers use the H-R
diagram to classify stars and to understand how stars
change over time.
Most of the stars in the H-R diagram form a diagonal line
called the main sequence. More than 90 percent of all stars,
including the Sun, are main-sequence stars. In the main
sequence, luminosity increases as surface temperature
increases.
The temperature scale along the bottom axis goes from
coolest on the right to hottest on the left. This is contrary to
the normal convention, where values increase going left to right on an axis. The temperature may be
replaced or supplemented with spectral class. The luminosity scale on the left axis is dimmest on the
bottom and gets brighter towards the top. This places the cooler, dimmer stars towards the lower right
and the hotter, more luminous stars at the upper left. Our own star, the Sun, is nearly in the middle of
both the temperature and luminosity scales relative to other stars. This puts it around the middle of the
main sequence.
Stars above the main sequence on the H-R diagram (higher luminosity), with the same temperature as
cooler main sequence stars, are larger. Also, stars that have the same luminosity as dimmer main
sequence stars, but are to the left of them (hotter) on the H-R diagram, are smaller. Bright, cool stars
are therefore necessarily very large. Similarly, stars that are very hot and yet still dim must be small.
On the H-R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are
below the main sequence.
Main-sequence stars are stars like the Sun but with different masses. The mass-luminosity relation
expresses a direct correlation between mass and luminosity for main-sequence stars. The greater the
mass of a main-sequence star, the greater its luminosity (and also the greater its radius and surface
temperature). In other words, stars that are higher up (brighter) on the main sequence are more
massive, larger, and hotter.
A nebula is a large cloud of gas and dust spread out in an immense volume. The birth of a star begins
when a disturbance, such as the shockwave from a supernova, triggers part of a nebula to collapse
inward. Gravity can then pull some of the gas and dust in the nebula together. The contracting cloud is
then called a protostar. A protostar is the earliest stage of a star’s life.
The inward collapse of material causes the center of the protostar to become very hot and dense. Once
the central temperature and density reach critical levels, about 10 million Kelvin, nuclear fusion
begins. During the fusion reaction, hydrogen atoms are combined together to form helium atoms.
When this happens, photons of light are emitted. Once the outward pressure created by the energy
given off during nuclear fusion balances the inward gravitational collapse of material, a state of
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hydrostatic equilibrium is reached, and the star no longer collapses. When this happens, the protostar
becomes a main-sequence star.
How long a star lives depends on its mass. Small-mass stars use their fuel more slowly than large-mass
stars, so they have much longer lives. When a star begins to run out of hydrogen fuel, the star becomes
a red giant or supergiant. When a star runs out of fuel, it becomes a white dwarf, a neutron star, or a
black hole.
When small-mass or medium-mass stars use up their fuel, their outer layers expand. At this stage they
are called red giants. Eventually, the outer parts grow bigger and drift into space, forming a cloud of
gas called a planetary nebula. The blue-white hot core of the star that is left behind cools and becomes
a white dwarf.
The Sun is an average medium-mass star. It has been shining as a stable main sequence star for about
5 billion years, and it should continue to shine steadily for another 5 billion years.
red giant
nebula
protostar
white dwarf
mainsequence
star
black dwarf
neutron star
supergiant
increasing mass
supernova
black hole
A dying supergiant star can suddenly explode. The explosion is called a supernova. After the star
explodes, some of the materials from the star are left behind. This material may form a neutron star.
Neutron stars are the remains of high-mass stars. They are even smaller and denser than white dwarfs.
In 1967, Jocelyn Bell found an object in space that appeared to give off regular pulses of radio waves.
Astronomers soon discovered that the source of the radio waves was a rapidly spinning neutron star.
Spinning neutron stars are called pulsars, short for pulsating radio sources.
The most massive stars become black holes when they die. After a large-mass star explodes, a large
amount of mass may remain. The gravity of the mass is so strong that gas is pulled inward, pulling
more gas into a smaller and smaller space. Eventually, the gravity becomes so strong that nothing can
escape, not even light.
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