Lecture 13 Light: the Cosmic Messenger Telescopes and Observational Astronomy

The Doppler Effect

Finding recession velocity the Doppler shift can be written: v =   c where = v is the recession velocity of the object  is the change in wavelength, - 0  is the wavelength in the rest frame

Doppler shift: example Remember that the H  line of Hydrogen has a rest wavelength of 0 = nm. In the star Vega, this line appears at a wavelength of = nm. –is Vega moving towards or away from us? –what is the radial velocity of Vega?

Another example: Hydrogen emits and absorbs photons with a wavelength of 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 Broadening

The 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 10 6 nm)/ peak = (2.9 x 10 6 )/(500 nm)  T = 5800 K

The luminosity of the Sun is 3.90 x W. Find the temperature of the Sun. this time we’re going to use the Stephan- Boltzman law: F = [5.7 x W/(m 2 x K 4 )] T 4 first we need to find the flux at the Sun’s surface. remember flux = energy/area so F sun = L sun /(4  R 2 sun ) R sun = 6.96 x 10 8 m  F = 6.41 x 10 7 W m -2 now we use T = (F/ 5.7 x W/(m 2 x K 4 )) 1/4  T = 5800 K

Other 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: 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 Astronomy

The Human Eye

How the eye works

How Cameras Work

There are two kinds of telescopes  Refracting  Refracting: a lens is used to focus the light from distant objects  Reflecting  Reflecting: a primary mirror is used to gather and focus light.

A Refracting Telescope A Refracting Telescope

A Reflecting Telescope

Alternative designs for reflecting telescopes

Charge Coupled Devices (CCD)

Fundamental 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 Separation  = 360 o. s /(2  d) “small angle formula”

Diffraction

The Diffraction Limit diffraction limit = 2.5 x 10 5 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 10 5 (500 x m/2.4 m) = 0.05 arcsec

Where 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 Kea

Basic Functions of Telescopes  Imaging/photometry  photometry involves accurate measurement of the light intensity  filters can be used to separate into different colors  spectroscopy  light spread out using a diffraction grating  time sequence  how an object’s brightness changes with time (supernovae, gamma ray bursts…)

A Basic Spectrograph

Spectral Resolution