Astronomy 100Exploring the Universe

Tuesday, Wednesday, Thursday

Tom Burbinetomburbine@astro.umass.edu

Magnitude System

• Brighter –lower number

http://www.astronomynotes.com/starprop/appmag.gif

4 Vestabrightest asteroid

Magnitude difference Relative intensity0 11 2.512 6.313 15.84 39.85 10010 104

15 106

Initially

• Everybody observed with their eyes

Figure 7.1

Figure 7.2a

Parallel light Lens

Figure 7.2b

Why are Telescopes better than your eyes?

• They can observe light in different wavelength regions (eyes can only see visible light)

• They can collect more light than eyes• They can be built to compensate for the distorting

effects of the atmosphere

Figure 7.6

Refracting telescope

Reflecting Telescope

Reflecting Telescopes

Resulting image inverted

All large modern telescopes are reflectors

• Since light passes through the lens of a refracting telescope,

• You need to make the lens from clear, high-quality glass with precisely shaped surfaces

It is

• Its easier to make a high-quality mirror than a lens

Also,

• Large lenses are extremely heavy

Also

• Lens focuses red and blue light slightly differently• Called chromatic aberration

http://en.wikipedia.org/wiki/File:Lens6a.svg

• Light changes speed as it moves from one medium to another

• The degree of bending of the light's path depends on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media.

• The refractive index of many materials (such as glass) varies with the wavelength of the light used

Also

• Light can be absorbed by the glass as it passes through the glass

• Minor problem for visible, but severe for ultraviolet and infrared light

Size of a telescope

• Diameter of its primary mirror or lens• Light collecting area is proportional to the

diameter squared since• Collecting area = π r2

• E.g., 8-meter telescope

• Telescope that took image b is twice as big as telescope that took image a

• Larger the telescope, more detail can be seen

a b

• Telescope on Mauna Kea (14,000 feet high)• Telescope is Japanese Subaru 8-m telescope

Atmosphere

• Atmosphere can absorb light• Atmosphere can scatter light• Atmosphere can distort light (twinkling)

Twinkling

• Twinkling of stars is caused by moving air currents in the atmosphere.

• The beam of light from a star passes through many regions of moving air while on its way to an observer’s eye or telescope.

• Each atmospheric region distorts the light slightly for a fraction of a second.

Advantages of space-based telescopes

• It can be open 24 hours, 7 days of week• Do not have to worry about distorting effects of

atmosphere • There is no extra background of light due to

scattering of light in the Earth’s atmosphere• Observe in more wavelength regions

Figure 7.20

http://www.scienzagiovane.unibo.it/English/radio-window/images/radiazioni-em.jpg

• Infrared light absorbed by molecules

http://www.ucar.edu/learn/1_3_1.htm

Not all light from a star reaches Earth

Light in space can be affected by dust

http://www.ipac.caltech.edu/2mass/outreach/survey.html

http://en.wikipedia.org/wiki/File:Rayleigh_sunlight_scattering.png

It does not help

• That you are closer to the stars

To measure light

• In the past, they used photographic plates• Now they use CCDs (charge-coupled devices)• CCD are electronic detectors• CCDs are chips of silicons

Figure 7.5

CCDs• CCDs convert light into electrons

William Boyle George Smith

Shared the 2009Physics Nobel Prizefor their discovery

How do they work?• The CCD is made up of pixels.• As the light falls on each pixel, the photons become electrons

due to the photoelectric effect. The photoelectric effect happens when photons of light hit the silicon of the pixel and knock electrons out of place.

• These electrons are then stored.• Essentially, the charge in each row is moved from

one site to the next, a step at a time. This has been likened to a “bucket row” or human chain, passing buckets of water down a line.

• As these buckets of electrons reach the end of the line they are dumped out and measured, and this analog measurement is then turned into a digital value.

• Thus, a digital grid is made which describes the image.

Color separation for digital cameras• Colored filters

CCDs

• CCDs can collect 90% of photons that strike them• Photographic plates can only collect 10% of the

photons• CCDs are split into squares called pixels• Data is in electronic form

Hubble Telescope

• Can observe in visible, infrared, and ultraviolet wavelength regions

• Named after Edwin Hubble, the father of modern cosmology

Hubble (launched in 1990)

Telescope is the size of a school bus

2.4 m mirror

Initially• Hubble’s primary mirror was polished to the

wrong shape • Was too flat at the edges• Was barely 2.3 micrometers out from the required

shape (1/50 the width of a human hair)• Images were not focused as well as they could be• Later shuttle mission fixed this problem by

installing a number of small mirrors

http://dayton.hq.nasa.gov/IMAGES/SMALL/GPN-2002-000064.jpg

Jupiter

Hubble replacement

• The first major components of the new James Webb Space Telescope are now being assembled.

• While Hubble is the size of a bus, the new James Webb will be the size of a jetliner.

• Will launch in 2014• James Webb is a former NASA administrator

during the Apollo program

• http://video.pbs.org/video/1456686369/

Any Questions?