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1IASS Tutorials 3: Stellar Colours, Distances and Magnitudes
1.
The human eye can see wavelengths of between about =350 nm (violet) and
=800 nm (red). To what frequency range does this correspond?
2.
A galaxy at a distance of 2x1022km from the sun emits radiowaves which are
detected on earth by a radio receiver which is tuned to 99 MHz. (a) What is the
wavelength of the detected signal? (b) How long ago did the observed events take
place before being detected on earth?
3.
The red giant star Betelguese has a surface temperature of T=3,400 K; while the
star Rigel in the same constellation of Orion, has a surface temperature of 10,100
K. Assuming ideal black-body spectra for both, at what wavelengths do the blackbody emission spectra for Betelguese and Rigel peak respectively? What colour
would each of these stars appear to the human eye?
4.
The Sun’s peak output wavelength is at 500nm. (a) What is the power output of
the sun per unit area? (b) Given that the solar radius = 7x108m and that distance
between the sun and earth = 1AU=1.5x1011m, what is the value of the SOLAR
CONSTANT ? (The solar constant is the light power from sun which is incident on
the earth, per unit area).
5.
The parallax angle of the star 61 Cygni was measured to be 0.316 seconds of
arc as measured from earth. What is the distance of 61 Cygni in units of (a)
parsecs; (b) light years; and (c) metres?
6.
Consider a model star which consists of a spherical black body with a surface
temperature of 28,000 K and a radius of 5.16x109m. Let this model star be located
at a distance of d=180pc from the earth. (This is a model of the star Dschubba in
the constellation of the Scorpois). Determine the following quantities for this star:
(a) Luminosity; (b) Asolute bolometric magnitude (M); (c) apparent bolometric
magnitude (m); (d) distance modulus; (e) radiant flux at the star’s surface; (f)
radiant flux at the earth’s surface; (g) peak wavelength,max.
(=5.67x10-8 Wm-2K-4; L0=4x1026W).