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