Chapter 27Chapter 27Stars and GalaxiesStars and Galaxies

Section 1Section 1

Characteristics of StarsCharacteristics of Stars

Notes 27-2Notes 27-2

Distance to StarsDistance to Stars

Light years: used to measure distance to Light years: used to measure distance to stars and galaxiesstars and galaxies– Distance that light travels in one yearDistance that light travels in one year– 300,000 km/s = speed of light300,000 km/s = speed of light– 9.5 trillion km in one year9.5 trillion km in one year– Sun in 8 light minutes from Earth Sun in 8 light minutes from Earth – Proxima Centauri: closest star to Earth (other Proxima Centauri: closest star to Earth (other

than the sun) is 4.2 light years awaythan the sun) is 4.2 light years awaySirius (brightest star): 9 lySirius (brightest star): 9 ly

Polaris: 700 lyPolaris: 700 ly

Distance to StarsDistance to Stars

Parallax: method used to determine the Parallax: method used to determine the distance to starsdistance to stars– Because the earth is moving the star appears Because the earth is moving the star appears

to shift in the sky depending on the time of to shift in the sky depending on the time of yearyear

– The closer the star is to the earth the greater The closer the star is to the earth the greater the shiftthe shift

– Astronomers use a photo to determine the Astronomers use a photo to determine the shift (photographed twice in a 6 month time)shift (photographed twice in a 6 month time)

Then calculate the distance to within 1000 lyThen calculate the distance to within 1000 ly

ParallaxParallax

Distance to StarsDistance to Stars

Astronomers compare the actual Astronomers compare the actual brightness and apparent brightnessbrightness and apparent brightness– Determines the distance from the earthDetermines the distance from the earth

Cepheid (Sef-EE-id) variable starCepheid (Sef-EE-id) variable star– Star’s brightness varies at a constant patternStar’s brightness varies at a constant pattern

Brightens and fades in a cycle that can be used to Brightens and fades in a cycle that can be used to determine how far away it isdetermine how far away it is

– Astronomers can then determine the distance Astronomers can then determine the distance to the galaxy it is in to the galaxy it is in

Stellar MagnitudesStellar Magnitudes

3 billion stars can be seen through 3 billion stars can be seen through telescopes on the surface telescopes on the surface – 6000 can be seen with the unaided eye6000 can be seen with the unaided eye

Over a trillion stars can be seen with the Over a trillion stars can be seen with the Hubble Space TelescopeHubble Space Telescope

Apparent Magnitude: brightness as it Apparent Magnitude: brightness as it appears from Earthappears from Earth

Absolute Magnitude: brightness as it Absolute Magnitude: brightness as it appears 32.6 ly awayappears 32.6 ly away

Apparent MagnitudeApparent Magnitude

The measurement of brightness is The measurement of brightness is assigned a number on a scaleassigned a number on a scale– Brightest stars have lowest numbersBrightest stars have lowest numbers– Dimmest stars have highest numbersDimmest stars have highest numbers

Most powerful telescopes can detect an Most powerful telescopes can detect an apparent magnitude of 29apparent magnitude of 29

Apparent MagnitudeApparent Magnitude

Unaided eye can detect a 6Unaided eye can detect a 6– Called a sixth magnitude starCalled a sixth magnitude star– First magnitude is one of the brightest in the First magnitude is one of the brightest in the

skysky

If the number is negative it is brighter If the number is negative it is brighter than the first magnitude starthan the first magnitude star– Sun (-26.8), Moon (-12.5), Venus (-4.6), Sun (-26.8), Moon (-12.5), Venus (-4.6),

Jupiter (-2.7), Sirius (-1.46)Jupiter (-2.7), Sirius (-1.46)

Absolute MagnitudeAbsolute Magnitude

Brightness from 32.6 lyBrightness from 32.6 ly

Ex: If the sun was 32.6 ly away, it would Ex: If the sun was 32.6 ly away, it would be a fifth magnitude star.be a fifth magnitude star.– Its absolute magnitude = +5Its absolute magnitude = +5

Most stars are between a -5 and +15Most stars are between a -5 and +15

Absolute MagnitudeAbsolute MagnitudeEach star has two magnitudesEach star has two magnitudes– Two depend on the distance the star is from Two depend on the distance the star is from

EarthEarth

Stars with a low apparent mag. and higher Stars with a low apparent mag. and higher absolute mag. appear brighter than if they absolute mag. appear brighter than if they were 32.6 ly awaywere 32.6 ly away– Like the SunLike the Sun

Stars with a high apparent mag. and low Stars with a high apparent mag. and low absolute mag.absolute mag.– Stars are too far away to look brightStars are too far away to look bright

Absolute MagnitudeAbsolute Magnitude

How far away is a star with apparent mag. How far away is a star with apparent mag. of +7 and absolute mag. of +7?of +7 and absolute mag. of +7?

32.6 light years32.6 light years

HR DiagramHR Diagram

Graph plotting the surface temperatures Graph plotting the surface temperatures versus the absolute magnitudes of starsversus the absolute magnitudes of stars

HR diagram (Hertzsprung-Russell)HR diagram (Hertzsprung-Russell)– Ejnar HertzsprungEjnar Hertzsprung– Henry RussellHenry Russell– Both discovered the patternBoth discovered the pattern

Brightness increases as the surface Brightness increases as the surface temperature increasestemperature increases

HR DiagramHR DiagramMain sequence starsMain sequence stars– Found in the band running down through the diagramFound in the band running down through the diagram– SunSun

Giants:Giants:– Large and coolLarge and cool– BetelgeuseBetelgeuse

Supergiants:Supergiants:– Very large and coolVery large and cool– AntaresAntares

White DwarfsWhite Dwarfs– Hot and smallHot and small– About the size of EarthAbout the size of Earth

HomeworkHomework

Sticky NotesSticky NotesKey Terms Ch. 27Key Terms Ch. 27

Due: Wednesday, Dec. 9Due: Wednesday, Dec. 9