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Stars Part One:. Brightness and Distance. Concept -1 – Temperature. λ max (metres) = 2.90 x 10 -3 m k T (Kelvin) λ max = Peak black body wavelength T = The star’s surface temperature in Kelvins. λmax = 2.90 x 10 -3 m K T. λ max. From Jay Pasachoff’s Contemporary Astronomy. - PowerPoint PPT Presentation
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Stars Part One:Stars Part One:Brightness and DistanceBrightness and Distance
Concept -1 – Temperature
λmax (metres) = 2.90 x 10-3 m kT (Kelvin)
λmax = Peak black body wavelengthT = The star’s surface temperature in Kelvins
λmax = 2.90 x 10-3 m KT
From Jay Pasachoff’s Contemporary Astronomy
λmax
Put this in your notes: A star has a surface temperature of 5787 K, what is its λmax?
λmax = (2.90 x 10-3 m K)/T = (2.90 x 10-3 m K)/5787 = 501 nm
501 nm
A star has a λmax of 940 nm, what is its surface temperature?
λmax = (2.90 x 10-3 m K)/T, T = (2.90 x 10-3 m K)/ λmax = (2.90 x 10-3 m K)/ (940E-9) = 3100 K
3100 K
Concept 0 – Total power output
Luminosity L = σAT4
L = The star’s power output in Wattsσ = Stefan Boltzmann constant = 5.67 x 10-8W/m2K4
A = The star’s surface area = 4πr2
T = The star’s surface temperature in Kelvins
Put this in your notes: Our Sun has a surface temp of about 5787 K, and a radius of 6.96 x 108 m. What is its Luminosity?
L = σAT4 = 5.67x10-8(4π(7x108 ) 2 )(5787)4 = 3.87x1026 Watts
3.87x1026 Watts
A star has a radius of 5.0 x 108 m, and Luminosity of 4.2 x 1026 Watts, What is its surface temperature?
Luminosity L = σAT4 , T =(L/(σ4(5x108)2)).25 =6968 K = 7.0x103 K
7.0x103 K
Concept 1 – Apparent Brightness
Apparent Brightness b = L 4πd2
b = The apparent brightness in W/m2
L = The star’s Luminosity (in Watts)d = The distance to the star
L is spread out over a sphere..
Apparent Brightness b = L/4πd2
The inverse square relationshipFrom Jay Pasachoff’s Contemporary Astronomy
Put this in your notes: Our Sun puts out about 3.87 x 1026 Watts of power, and we are 1.50 x 1011 m from it. What is the Apparent brightness of the Sun from the Earth?
b = L/4πd2 = 3.87E26/ 4π1.50E112
= 1370 W/m2
1370 W/m2
Another star has a luminosity of 3.2 x 1026 Watts. We measure an apparent brightness of 1.4 x 10-9 W/m2. How far are we from it?
b = L/4πd2 , d = (L/4πb).5 = 1.3x1017 m
1.3x1017 m
Concept 2 – Apparent Magnitude
Apparent Magnitude:
b = The apparent brightness in W/m2
m = The star’s Apparent MagnitudeNote that the smaller b is, the bigger m is.Logarithmic scale: x100 in b = -5 in m
€
m = 2.5log10
2.52x10−8 W
m2
b
⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟
Put this in your notes: What is the apparent magnitude of a star with an apparent brightness of 7.2x10-10 Wm-2? What is that of a star with an apparent brightness of 7.2x10-12 Wm-2?
3.9, 8.9
From Jay Pasachoff’s Contemporary Astronomy
So apparent magnitude is a counter-intuitive scale
-27 is burn your eyes out,
1 is a normal bright star, and
6 is barely visible to a naked, or unclothed eye. (You can’t see anything if you clothe your eyes!!)
A magnitude 1 star delivers 100 times more W/m2 than a magnitude 6 star.
What is the Apparent Magnitude of a star that has an apparent brightness of 1.4 x 10-9 W/m2 ?
m = 2.5log10 (2.52 x 10-8W/m2/b) = 2.5log10 (2.52 x 10-8W/m2/ 1.4 x 10-9 W/m2) = 3.1
3.1
The Hubble can see an object with an apparent magnitude of 28. What is the apparent brightness of such a star or galaxy?
m = 2.5log10 (2.52 x 10-8W/m2/b) , b = 2.52 x 10-8W/m2 /10(m/2.5)
= 1.6 x10-19 W/m2
1.6 x10-19 W/m2
Concept 3 – Absolute Magnitude
Absolute Magnitude: m - M = 5 log10(d/10)
M = The Absolute Magnituded = The distance to the star in parsecsm = The star’s Apparent Magnitude
The absolute magnitude of a star is defined as what its apparent magnitude would be if you were 10 parsecs from it.
Absolute Magnitude: m - M = 5 log10(d/10)M = The Absolute Magnituded = The distance to the star in parsecsm = The star’s Apparent Magnitude
Example: 100 pc from an m = 6 star, M = ?(10x closer = 100x the light = -5 for m)
M = 6 - 5 log10(100/10) = 1
Put this in your notes: The Sun has an apparent magnitude of -26.8, we are 1.5x108 km or 4.9 x 10-6 pc from the sun. What is the sun’s absolute magnitude?
M = m - 5 log10(d/10) = -26.8 - 5 log10(4.86 x 10-6 /10) = 4.7
4.7
You are 320 pc from a star with an absolute magnitude of 6.3. What is its apparent magnitude?
M = m - 5 log10(d/10), m = M + 5 log10(d/10) = 6.3 + 5 log10(320/10) = 14
14
Concept 4 – H-R diagrams
In 1910, Enjar Hertzsprung of Denmark, and Henry Norris Russell at Princeton plotted M vs T in independent research.
From Douglas Giancoli’s Physics
Main Sequence
Hot Cooler
Dim
Bright
From Douglas Giancoli’s Physics
Hot Cooler
Dim
Bright
From Jay Pasachoff’s Contemporary Astronomy
O Oh (Hot)B beA aF fineG girl,K kissM me! (Cooler)
The Star
The atmosphere
Light from star is filtered by atmosphere
From Jay Pasachoff’s Contemporary Astronomy
Spectral types:
Suffixes 0 (hottest) - 9 (coolest) so O0, O1…O9, then B0, B1…
O -
B -
A -
F -
G -
K -
M -
30,000 - 60,000 K, ionized H, weak H lines, spectral lines arespread out. O types are rare and gigantic.10,000 - 30,000K, H lines are stronger, lines are less spread out(Rigel, Spica are type B stars)7,500 - 10,000K, strong H lines, Mg, Ca lines appear (H and K)(Sirius, Deneb and Vega are A type stars)6,000 - 7,500K, weaker H lines than in type A, strong Ca lines (Canopus (S.H.) and Polaris are type F)5,000 - 6,000K, yellow stars like the sun. Strongest H and Klines of Ca appear in this star.3,500 - 5,000K, spectrum has many lines from neutral metals.Reddish stars (Arcturus and Aldebaran are type K stars)3,500 or less, molecular spectra appear. Titanium oxide linesappear. Red stars (Betelgeuse is a prominent type M)
SO…
Hot stars are:Big, Bright, Brief and Blue
Cool stars are: Diminuitive, Dim, and Durable and um… reD
More about brief and durable next time…
Dim
Bright
Spectroscopic “parallax”
Spectrum = M(from diagram)Measure mFind d
From Douglas Giancoli’s Physics
Put in your notes: How far is an B0 that has an m of 8?
M = m - 5 log10(d/10)5 log10(d/10) = m - Mlog10(d/10) = (m - M)/5d/10 = 10((m - M)/5)
d = (10 pc)10((8 - -4.3)/5)
d = 2884 pc
2884 pc
How far is an K0 that has an m of 14?
d = (10 pc)10((14 -
5.8)/5)
d = 437 pc
437 pc