lecture 13
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Lecture 13. Light: the Cosmic Messenger Telescopes and Observational Astronomy. The Doppler Effect. Finding recession velocity. the Doppler shift can be written: v = Dl/l 0 c where = v is the recession velocity of the object Dl is the change in wavelength, l - l 0 - PowerPoint PPT PresentationTRANSCRIPT
Lecture 13 Lecture 13
Light: the Cosmic Messenger
Telescopes
and Observational Astronomy
The Doppler EffectThe Doppler Effect
Finding recession velocityFinding recession velocity
the Doppler shift can be written:
v = c
where = v is the recession velocity of the object
is the change in wavelength, -0is the wavelength in the rest frame
Doppler shift: exampleDoppler shift: example
• Remember that the H line of Hydrogen has a rest wavelength of 0 = 656.285 nm. In the star Vega, this line appears at a wavelength of = 656.255 nm. – is Vega moving towards or away from us?– what is the radial velocity of Vega?
Another example:Another example:
Hydrogen emits and absorbs photons with a wavelength of 21.12 cm, in the radio. This famous line is called the 21-centimeter line. The galaxy NGC3840 is moving away from us at a speed of 7370 km/s. At what wavelength would we expect to detect the 21-cm line from this galaxy?
Doppler BroadeningDoppler Broadening
The Sun as a BlackbodyThe Sun as a Blackbody
• The peak wavelength of the Sun’s light is about 500 nm. What is the surface temperature of the Sun?
• we can use Wien’s law:
T = (2.9 x 106 nm)/peak
= (2.9 x 106 )/(500 nm)
T = 5800 K
• The luminosity of the Sun is 3.90 x 1026 W. Find the temperature of the Sun.
• this time we’re going to use the Stephan-Boltzman law:F = [5.7 x 10-8 W/(m2 x K4)] T4
first we need to find the flux at the Sun’s surface. remember flux = energy/area so
Fsun = Lsun/(4R2sun)
Rsun = 6.96 x 108 m
F = 6.41 x 107 W m-2
• now we use T = (F/ 5.7 x 10-8 W/(m2 x K4))1/4
T = 5800 K
Other Stars and our SunOther Stars and our Sun
• Sirius is the brightest star in the night sky. It appears blue and its peak flux is at 280 nm, in the UV.– is Sirius hotter or cooler than our Sun? What is
its temperature?– compare the energy flux at the surface of Sirius
with that at the surface of our Sun.
Summary:Summary:
• spectra consist of continuum, emission lines, and absorption lines
• by studying the spectra of distant objects we can learn about their composition, surface temperature, radial velocity, and internal velocity.
Telescopes and Observational Telescopes and Observational AstronomyAstronomy
The Human EyeThe Human Eye
How the eye worksHow the eye works
How Cameras WorkHow Cameras Work
There are two kinds of telescopesThere are two kinds of telescopes
RefractingRefracting: a lens is used to focus the light from distant objects
ReflectingReflecting: a primary mirror is used to gather and focus light.
A Refracting TelescopeA Refracting Telescope
A Reflecting TelescopeA Reflecting Telescope
Alternative designs for reflecting telescopesAlternative designs for reflecting telescopes
Charge Coupled Devices (CCD)Charge Coupled Devices (CCD)
Fundamental Telescope PropertiesFundamental Telescope Properties
Light collecting area (diameter of primary mirror or lens)
angular resolution (smallest angular distance that can be resolved clearly)diffraction limit (limitation on angular
resolution due to light diffraction) -- depends on diameter of primary and wavelength of light being observed
limited by effects of Earth’s atmosphere
Angular SeparationAngular Separation
= 360o . s /(2d)
“small angle formula”
DiffractionDiffraction
The Diffraction LimitThe Diffraction Limit
diffraction limit = 2.5 x 105 x wavelength of light(arcsec) diameter of telescope
Find the diffraction limit of the 2.4 m Hubble Space Telescope for visible light (500 nm).
d.l. = 2.5 x 105 (500 x 10-9 m/2.4 m) = 0.05 arcsec
Where to put your telescopeWhere to put your telescope
high and dry – to minimize the blurring effects of the Earth’s atmosphere and emission/absorption from water vapor
away from light pollution with roads, electricity, and other support
systems nearby
The Summit of Mauna KeaThe Summit of Mauna Kea
Basic Functions of TelescopesBasic Functions of TelescopesImaging/photometry
photometry involves accurate measurement of the light intensity
filters can be used to separate into different colors
spectroscopylight spread out using a diffraction grating
time sequencehow an object’s brightness changes with time
(supernovae, gamma ray bursts…)
A Basic SpectrographA Basic Spectrograph
Spectral ResolutionSpectral Resolution