astronomy project report.pdf
TRANSCRIPT
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Applied Physics Department
Fundamentals of Astronomy and Space Sciences
Spectra of Stars (OBAFGKM)
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Abstract:
In this project we learned about stars and the different spectra types of starsand how they are related to their surface temperatures. We attempted to obtain the
spectrum of several stars in order to determine their spectral type (OBAFGKM)
and therefore to estimate their surface temperatures, using SCASS (Sharjah Centerof Astronomy and Space Sciences) observatory’s telescope.
Theory:
In astronomy, stars are usually classified
according to their spectral characteristics. This is
done by analyzing the light that’s coming from a
star into a spectrum with specific absorption lines(Figure 1). Spectra as such can be used to extract
information about the photosphere of stars like their
chemical composition, temperature and the density.
Usually, the laws by which spectra are described are given by Kirchoff’s Laws:
1.
A hot opaque body (dense gas or a solid) produces a continuous
spectrum.
2.
A hot transparent gas produces an emission line spectrum.3. A cool, transparent gas produces an absorption line spectrum.
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The classifications of stars according to their temperatures are seven
categories (OBAFGKM). Spectra of types O and B have few absorption lines
indicating their simplicity in terms of atomic structure which is associated with
high temperatures. On the other hand, spectral types K and M have a much more
absorption lines that mean the large number of possible atomic structures and
therefore indicating lower temperatures (Figure 2).
The continuous spectra of stars can tell us about the apparent colour of a star because a star approximates a black body. Continuous spectra for stellar interiors at
different temperatures are described by Planck Curves shown in the figure to the
left. Note that as the temperature
increases the total amount of light
energy produced (the area under the
curve) increases and the peak
wavelength (the color at which the
most light is produced) moves tosmaller more energetic wavelengths.
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Class Effective
Temperature
Conventional
colour
description
Hydrogen
lines
Maximum
Wavelength
O 30,000 K Blue Weak 73 nm
B 10,000~30,000 K Blue white Medium 145 nmA 7,500~10,000 K White Strong 290 nm
F 6,000~7,500 K Yellow white Medium 387 nm
G 5,200~6,000 K Yellow Weak 527 nm
K 3,700~5,200 K Orange Very weak 725 nm
M 2,400~3,700 K Red Very weak 966 nm
Class O:
O-type stars are very hot and extremely luminous, with most of their radiatedoutput in the ultraviolet range. These are the rarest of all main-sequence stars.
About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar
neighbourhood are O-type stars.
Class B:
B-type stars are very luminous and blue. As O- and B-type stars are so
energetic, they only live for a relatively short time. About 1 in 800 (0.125%) of the
main-sequence stars in the solar neighbourhood are B-type stars.
Class A:
A-type stars are among the more common naked eye stars, and are white or
bluish-white. They have strong hydrogen lines, at a maximum by A0, and also
lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5.
Class F:
F-type stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, CrI) beginning to gain on ionized metal lines by late F. Their spectra are
characterized by the weaker hydrogen lines and ionized metals. Their color is
white. About 1 in 33 (3.03%) of the main-sequence stars in the solar
neighbourhood are F-type stars.
Class G:
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G-type stars, including the Sun have prominent H and K lines of Ca II, which
are most pronounced at G2. They have even weaker hydrogen lines than F, but
along with the ionized metals, they have neutral metals. Class G main-sequence
stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the
solar neighbourhood.
Class K:
K-type stars have an orange colour and are slightly cooler than the Sun. They
make up about 12%, nearly one in eight, of the main-sequence stars in the solar
neighbourhood. They have extremely weak hydrogen lines, if they present at all.
Class M:
Class M stars are by far the most common. About 76% of the main-sequencestars in the solar neighbourhood are class M stars. However, because main-
sequence stars of spectral class M have such low luminosities, none are bright
enough to be visible to see with the unaided eye unless under exceptional
conditions.
Results:
1.
Rigel (Spectrum type: B8)
Figure 2: Using the blue filter Figure 1: No filters
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2.
Algol (Spectrum type: B8)
3. Betelgeuse (Spectrum type M2)
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4.
Deneb (Spectrum type A2)
5.
Hamal (Spectrum type K2)
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6.
Menkar (Spectrum type M2)
7. Mirphak (Spectrum type F5)