astronomy project report.pdf

7/23/2019 Astronomy Project Report.pdf

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Applied Physics Department

Fundamentals of Astronomy and Space Sciences

Spectra of Stars (OBAFGKM)



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.



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.



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:

https://en.wikipedia.org/wiki/Iron

https://en.wikipedia.org/wiki/Magnesium

https://en.wikipedia.org/wiki/Silicon

https://en.wikipedia.org/wiki/Calcium


https://en.wikipedia.org/wiki/Chromium

https://en.wikipedia.org/wiki/Chromium



https://en.wikipedia.org/wiki/Silicon

https://en.wikipedia.org/wiki/Magnesium




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

https://en.wikipedia.org/wiki/Sun



https://en.wikipedia.org/wiki/Sun



2.

Algol (Spectrum type: B8)

3. Betelgeuse (Spectrum type M2)



4.

Deneb (Spectrum type A2)

5.

Hamal (Spectrum type K2)



6.

Menkar (Spectrum type M2)

7. Mirphak (Spectrum type F5)

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