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U N D E R S T A N D I N G
STELLAR TEMPERATURE
This temperature is primarily
influenced by the star's mass and age,
with hotter stars appearing blue
and cooler stars appearing red.
Understanding stellar temperature is
essential for astronomers as it helps
in classifying stars and deciphering
their life cycles and evolutionary
stages.
Imagine you have a light that is
just ever so slightly on. Notice
that if you increase the current, the
brightness increases, and it also
changes in color. It went from a
yellowish, orangey color to a
more white color.
what else changes? The
B ut
temperature also increases.
As it gets brighter and changes from
orange to white, the temperature
seems to be increasing. Now, we seem
to have a relationship between
intensity and temperature, but also
between color and temperature.
Stellar temperature refers to the
surface temperature of a star,
typically measured in Kelvin, and it
plays a crucial role in determining the
star's color and spectral type.
H
ow do astronomers actually
measure a star's surface
temperature?
Measuring
a
star's
surface
temperature is easier than measuring
luminosity because temperature
doesn't change with distance. We
determine surface temperature from
either the star's color or its
spectrum.
Stars appear in different colors
because
they
emit
thermal
radiation. A hot star like Vega emits
more blue lig
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more blue light, and a cooler star like
Arcturus emits more red light.
Astronomers can measure surface
temperature by comparing a star's
apparent brightness in two
different colors of light. For example,
consider the hot star Vega. If we
compare the amount of light through
a red filter with the amount of light
through a blue filter, there will be
more light in the blue because the
star is hot, and hotter objects emit
more light at shorter wavelengths.
A star's spectrum provides a
second and more accurate way to
measure
surface
temperature.
Spectra for hot stars look different
from the spectra of cool stars. The
types of spectral lines present in a
star's spectrum can provide us a
direct measure of the star's surface
temperature.
ut
before
look
at
from
a
spectroscopy perspective, we
need to look at what we refer to
as a blackbody curve.
B temperature
we
What this really represents are all the
wavelengths coming off a star, which
in essence is what we refer to as a
blackbody.
A star does not give the same
intensity for every wavelength that
comes off it. In this example, the
maximum wavelength we get is in
the roughly green/yellow range of
the spectrum. Down to the right,
you'll see we have some red light,
but it's less intense. There were also a
lot of infrared coming off, but it's
significantly less in proportion to the
light that's coming off. If we keep
going this way, we would also get
microwaves and radio waves. There’s
also ultraviolet coming off, but it
drops off significantly.
So, all wavelengths are given off but
at varying intensities. If you look at
the wavelength with the greatest
intensity, we can determine the
temperature of the body. This leads
us to Wien's Law. Wien's Law, in
essence, says that temperature is
equal to a constant, 0.00289,
divided by the wavelength that has
the greatest intensity.
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O-type Stars
Temperature: ≥ 30,000 K
Definition: Very hot and extremely
luminous. Because they are so
massive, they have very hot cores and
burn through their hydrogen fuel
very quickly making them the first
stars to leave the main sequence (the
rarest of all main-sequence stars).
For example, if a star has a blackbody
curve looking like this, its wavelength
has a temperature of 3500 Kelvin.
The next star above it is hotter, and
you'll see that the wavelength gets
smaller, which is consistent with
Wien's Law. It shows an inverse
relationship: the hotter the star, the
further the peak moves toward
the left. That is, as the temperature
increases, the wavelength decreases.
If we then analyze all the stars we can
see and use the blackbody radiation
curves, we can classify stars
according to various spectral
classes.
The hottest stars, with the bluest
colors, are called spectral type O. The
next hottest are spectral type B,
followed by A
A, F, G, K, and M, with
spectral type M being the coolest.
Here
are
mnemonics
for
remembering the spectral types.
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B-type Stars
Temperature: 10,000 - 30,000 K
Definition: B-type stars are very
luminous and blue. Their spectra
have neutral helium lines, which are
most prominent at the B2 subclass,
and moderate hydrogen lines.
A-type Stars
Temperature: 7,500 - 10,000 K
Definition: Among the more
common naked eye stars; white or
bluish-white. They have strong
hydrogen lines, at a maximum by A0,
and also lines of ionized metals.
F-type Stars
Temperature: 6,000 - 7,500 K
Definition: F-type stars are yellowwhite. While they have lifespans a bit
shorter than the Sun, they are likely
candidates for planets and life with
habitable zones around 1.5 AU away.
G-type Stars
Temperature: 5,200 - 6,000 K
Definition: G-type stars, like our
Sun (a G2V star), are yellow in color.
Their long lifespans and relatively
high brightness means they are very
good candidates for life-bearing
planets.
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K-type Stars
Temperature: 3,700 - 5,200 K
Definition: Compared to stars like
the Sun, they are generally notably
smaller, cooler, less massive, and less
luminous. The most luminous are
about two-thirds the luminosity of
the Sun.
M-type Stars
Temperature: 2,400 - 3,700 K
Definition: They are by far the
most common type of luminous star,
making up roughly 76% of all main
sequence stars. They have generally
not good conditions for intelligent
life.
The spectral type letters may
seem a little random. When
astronomers first began observing
stars through telescopes, they
initially classified them based on
color. Stars were grouped into broad
color categories: white, yellow,
red, and deep red. This colorbased classification system was soon
refined into a more detailed scheme,
assigning letters to specific color
ranges. White stars were labeled A to
D, yellow stars were categorized as E
to L, and red stars were marked M
and N.
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Then,
astronomers
shifted
to
classifying
stars
by
surface
temperature, finding it more
systematic. However, the original
letter-based system was kept, leading
to the Harvard spectral classification.
This system organizes stars from
hottest (O-type) to coolest (M-type)
and was developed by early
astronomer Annie Jump Cannon.
When we examine a star's spectrum,
we notice dark lines called
"absorption lines." These lines
form because elements in the star’s
outer layers absorb certain colors of
light.
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For example, hydrogen absorbs light
at specific wavelengths, creating a
unique set of lines.
In the hottest stars, like O stars,
hydrogen’s absorption lines are weak.
This happens because most of the
hydrogen atoms are stripped of their
electrons due to the intense heat, so
they can't absorb light in the usual
way. However, helium, which can
hold onto at least one electron in
these conditions, still shows some
absorption lines, helping us detect its
presence.
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For each letter class, the number 0
represents the hottest stars within
that class, while the number 9
represents the coolest stars within
the same class. As you move from 0
to 9 within a class, the temperature
gradually decreases, and the colors
shift slightly.
A spectral type O0 star is hotter than
an O1, which is hotter than an O9.
The O9 star is hotter than the B0
star, and so on. The coolest is M9.
Sun is a type G2 star - hotter
O ur
than a G3 but cooler than a G1.
As we move down the temperature
scale to slightly cooler stars, like A
stars, hydrogen absorption lines
become stronger because the cooler
temperature allows hydrogen atoms
to retain their electrons. This pattern
of absorption lines helps astronomers
identify both the temperature and the
composition of a star.
In addition to the main spectral
classes, astronomers realized that
they needed more detail to describe
the wide range of characteristics
within each class, so they introduced
a numerical subdivision from 0 to
9 for each letter class, allowing for
more precise categorization of stars
based on their temperature and color.
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REFERENCES:
1
2
3
Professor,D(2018, August 10)
Classification
of
Starshttps://www.youtube.com/
watch?v=Y5VU3Mp6abI
Scott,M(2018,April21)Where
did the star types come from?
https://www.youtube.com/watc
h?v=oM5lEG2woA0
Stellar Temperature: Techniques
& Definitions | VaiA. (n.d.). Vaia.
https://www.vaia.com/enus/explanations/physics/astrop
hysics/stellar-temperature/
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