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Unformatted text preview: Stellar Properties I. COMPOSITION: What elements are present? 1. Take spectrum. 2. Compare spectral lines to lines of wavelengths
measured for diﬁerent elements in laboratory.
Results: Stars are mainly hydrogen and helium, but
contain other elements in small quantities. 2. TEMPERATURE: How hot? 1. Take spectrum. 2. Scan for most intense wavelength (peak 3.). 3. Use Wien’s law (T a. 1/7») to calculate. Results: Stats range in surface temperature ﬁ'om 3000 K
to 100,000K. Astronomers use a classiﬁcation system for stars based on
temperature—spectral type. The temperature of a star
greatly inﬂuences the dominance of certain spectral lines.
Stars with surface temperature of l0,000 K have strong
(visible) hydrogen lines. For stars like the Sun (5800 K)
ionized calcium lines appear stronger than the hydrogen
lines—this doesn’t mean that there is more calcium than
hydrogen in the Sun. The brightness of spectral lines
depends on temperature. Cool stars (3 000 K) have
spectral features associated with molecules present. . . Spectra reveal temperature
" and composition 2 Analysis of spectral lines gives _ composition (H, He) but temperature
inﬂuences line dominance (1) Wien's law—“peak” wavelength of
emission indicates temperature (2) ; Spectral classiﬁcation system was _
developed in the early 1900s 7 3. LUMINOSITY: How much energy is radiated? The energy emitted every second by a star depends on its temperature (Stefan’s law [E a T4] gives the energy
radiated per square centimeter) and its size (ﬁrom radius
you can calculate the number of square centimeters of
surface area). This isn’t quite so siraight forward; since what we
observe on Earth depends on the distance to the star.
Here what astronomers do: 1. Take spectrum. 2. Classify the star (OBAFGKM) and from the width
of the spectral lines—estimate its luminosity class
(indicates whether this is a large or small star). This
analysis will allow you to estimate the star’s absolute
magnitude (M)——-how bright the star would appear at a
distance of 10 parsecs (32.5 LY). 3. This analysis allows astronomers to compare the
amount of energy radiated so that they can be compared. Results: There are stars emitting one millionth the Sun’s energy to a few million times the Sun’s energy. Peak u'sible #2 Visible peak is
wavelength is red 1 yellow
500 mm rm now she m 1500 we
mm ma huh." Us. From the peak wavelength,
the temperature is
determined \‘ isible peak is blue
i .i/ 40—25—5145 —io —s ii iii». is 29 as'
Appmmmngmmmv) . cram“! Pin-4mg cm.“ u Apparent magnitude is how bright an object appears
from Earth (observation) A smaller magnitude indicates a brighter object. i Sirius looks bright because it is
relatively close, 8 LY away a Spica looks bright because it is a
powerhouse—260 LY away but 100
times more luminous than Sirius There is a relationship between distance
luminosity (spectrum), and how bright
star appears(observation) from Earth. Annie Cannon 8: others
scrutinized 350,000 spectra Systematic progression in ‘
appearance of spectra (Lab #8) due to
surface temperature OBAFGKM—Oh, be a ﬁne guy/gal
kiss me a Sun is a 62 star a (F9, GO, G1, 62, ...GS, K0, K1, etc.) 4. DISTANCE—How far away is it?
4A. DISTANCE—Stellar Parallax
1, Observe the positions of stars over a year. 2. Measure parallax angle (p). 3. Use formula to solve for distance, d = 1/p. d in par-secs, p in arcseconds Results: This method only works for stars <1000 parsecs
(Hipparcos satellite). Actually from the ground it is more
like < 50 parsecs/due to atmos heric blurrin . 4B. DISTANCE—Spectroscopic Parallax
ii Take spectrum.
2. Classify spectral type and luminosity class. This is
most easily expressed as absolute magnitude, M.
3. Measure the apparent magnitude (m) using a telescope.
The apparent magnitude is how bright a star appears from
the Earth. Very easy to measure!
4. Use the equation relating apparent and absolute
magnitudes to distance. Remember: the smaller the
magnitude value, the brighter the star. m — M =5 log d— 5 solve for distance, d m — M is called the distance modulus
Results: A distance can be estimated if the star is bright enough to get good spectral information, This method is
used for their/est majority of stars in our galaxy HR Diagram Absolute
Magnitude £2 @30 A0 _F, I Spectral type
lo; 000 V- IIgawiL . I -
5200K Surface temperature ’ Km 3300K Using trigonometry to
determine distancis is used
by surveyou on Earth. 1 (4a) The most accurate distances nre
determined using stellar parallax.
(Direct observations) u mum ) (4b)
m — M = 5 log d - 5
v; m = apparent magnitude, observe ; M = absolute magnitude (3 measure
of luminosity), it is the apparent
magnitude 3 star has at the standard distance of 10 EC. from spectrum
spectral type (Lab 8) (m-M) is called the distance modulus 54% H would the Sun
be at a distance of 10 pc? .41 Only 1 AU away, the Sun’s apparent g
magnitude is ~26] 7 '
z 1 Solar luminosity = 2 x 102‘ Watts 3 But at a distance of 10 pc (32.5 LY) the
Sun’s apparent magnitude would only
be +4.8 FAINT! ' "-3 Brightest stars have negative
absolute magnitude values How do stars compare? 5 1910 Hertzsprung & Russell
independently plottted similar graphs
1 A graph is a comparison—here
stellar luminosity & temperature
(absolute magnitude & spectral type) Stars are not randomly
é scattered on HR Diagram a Sun’s coordinates: G2, 1 L9
(5800 Kelvins; M=+4.8)
3 Most stars lie on the main sequence
3 Also ﬁnd red giants, white dwarfs,
and supergiants ...
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This note was uploaded on 05/18/2008 for the course ASTR 101 taught by Professor Deming during the Spring '07 term at Maryland.
- Spring '07