2. How near is the closest star other than theHow near is the closest star other than the
Sun?Sun?
Is the Sun brighter than other stars, or justIs the Sun brighter than other stars, or just
closer?closer?
What colors are stars?What colors are stars?
Are brighter stars hotter?Are brighter stars hotter?
What sizes are stars?What sizes are stars?
Are most stars isolated from other stars,Are most stars isolated from other stars,
as the Sun is?as the Sun is?
WHAT DO YOU THINK?WHAT DO YOU THINK?
3. Apparent Magnitude Scale – brightnessApparent Magnitude Scale – brightness
of a star as seen from Earthof a star as seen from Earth
Several stars in and around the
constellation Orion labeled with
their names and apparent
magnitudes
Astronomers give the
brightness of objects in the sky by
apparent magnitudes. Stars visible
to the naked eye have magnitudes
between m = –1.44 and about
m = +6.
4. The Inverse-Square LawThe Inverse-Square Law
The farther a star is from Earth, the dimmer it looks
to us. Doubling the distance makes the star look
one-fourth as bright. Tripling the distance
decreases the star’s brightness by a factor of 9.
5.
6. Absolute Magnitude – the actualAbsolute Magnitude – the actual
brightness of a starbrightness of a star
Absolute magnitude tells how bright a starAbsolute magnitude tells how bright a star
really is, no matter how far from Earth it is.really is, no matter how far from Earth it is.
Are the car lights actually dimmer as theAre the car lights actually dimmer as the
car moves away?car moves away?
7.
8. No. Their actual brightness (absoluteNo. Their actual brightness (absolute
magnitude) is the same no matter themagnitude) is the same no matter the
distance.distance.
But theyBut they looklook dimmer (apparentdimmer (apparent
magnitude) to us when the car is farthermagnitude) to us when the car is farther
away.away.
9.
10. Temperature and Color (review)Temperature and Color (review)
Hottest = blue color
Medium = orange/yellow color
Coolest = red color
11. “Oh, Be A Fine Guy/Girl, Kiss Me!”
Spectral Classes (Color and Temperature)Spectral Classes (Color and Temperature)
14. Hertzsprung-Russell (HR)Hertzsprung-Russell (HR)
DiagramDiagram
Star brightness is plotted against star spectralStar brightness is plotted against star spectral
types (color / temperature).types (color / temperature).
Brightness and spectral type are related.Brightness and spectral type are related.
Main-sequence stars (fusing hydrogen toMain-sequence stars (fusing hydrogen to
helium) fall along the red curve.helium) fall along the red curve.
Giants are to the upper right and super-giantsGiants are to the upper right and super-giants
are on the top.are on the top.
White dwarfs are below the main sequence.White dwarfs are below the main sequence.
16. Star Size Is Also ImportantStar Size Is Also Important
Hotter stars are brighter than cooler starsHotter stars are brighter than cooler stars
(of the same size).(of the same size).
Bigger stars are brighter than smaller starsBigger stars are brighter than smaller stars
(of the same temperature).(of the same temperature).
So the brightest stars are the biggest,So the brightest stars are the biggest,
hottest ones.hottest ones.
L = RL = R22
TT44
(L = brightness, R = radius,(L = brightness, R = radius,
T = temperature)T = temperature)
17. Each dot = a main-Each dot = a main-
sequence star.sequence star.
The dot’s number isThe dot’s number is
the mass of thatthe mass of that
star in solar massesstar in solar masses
(Sun = 1).(Sun = 1).
Mass, brightness,Mass, brightness,
and temperature ofand temperature of
main-sequencemain-sequence
stars increase fromstars increase from
lower right to upperlower right to upper
left.left.
Mass-Temperature-BrightnessMass-Temperature-Brightness
18. WHAT DID YOU THINK?WHAT DID YOU THINK?
How near is the closest star other than the Sun?How near is the closest star other than the Sun?
Proxima Centauri is about 40 trillion kilometers (25 trillionProxima Centauri is about 40 trillion kilometers (25 trillion
miles) away. It takes light about 4 years to reach themiles) away. It takes light about 4 years to reach the
Earth from there.Earth from there.
How luminous is the Sun compared with other stars?How luminous is the Sun compared with other stars?
The most luminous stars are about a million timesThe most luminous stars are about a million times
brighter and the least luminous stars are about abrighter and the least luminous stars are about a
hundred thousand times dimmer than the Sun.hundred thousand times dimmer than the Sun.
What colors are stars?What colors are stars?
Stars are found in a wide range of colors, from redStars are found in a wide range of colors, from red
through violet, as well as white.through violet, as well as white.
19. Are brighter stars hotter than dimmer stars?Are brighter stars hotter than dimmer stars?
Not necessarily. Many brighter stars, such as red giants,Not necessarily. Many brighter stars, such as red giants,
are cooler but larger than hotter, dimmer stars, such asare cooler but larger than hotter, dimmer stars, such as
white dwarfs.white dwarfs.
What sizes are stars?What sizes are stars?
Stars range from more than 1000 times the Sun’sStars range from more than 1000 times the Sun’s
diameter to less than 1/100 the Sun’s diameter.diameter to less than 1/100 the Sun’s diameter.
Are most stars isolated from other stars, as the Sun is?Are most stars isolated from other stars, as the Sun is?
No. In the vicinity of the Sun, two-thirds of the stars areNo. In the vicinity of the Sun, two-thirds of the stars are
found in pairs or larger groups.found in pairs or larger groups.
WHAT DID YOU THINK?WHAT DID YOU THINK?
Editor's Notes
FIGURE 11-2 Apparent Magnitude Scale (a) Several stars in
and around the constellation Orion are labeled with their names
and apparent magnitudes. For a discussion of star names, see
Guided Discovery: Star Names. (b) Astronomers denote the
brightnesses of objects in the sky by their apparent magnitudes.
Stars visible to the naked eye have magnitudes between
m = –1.44 (Sirius) and about m = +6. CCD (charge-coupled
device) photography through the Hubble Space Telescope or a
large Earth-based telescope can reveal stars and other objects
nearly as faint as magnitude m = +30. (a: Okiro Fujii, L’Astronomie)
FIGURE 11-3 The Inverse-Square Law (a) This drawing
shows how the same amount of radiation from a light source
must illuminate an ever-increasing area as the distance from the
light source increases. The decrease in brightness follows the
inverse-square law, which means, for example, that tripling the
distance decreases the brightness by a factor of 9.
FIGURE 11-4 Temperature and Color (a) This beautiful Hubble Space Telescope image shows the variety of
colors of stars. (b) This diagram shows the relationship between the color of a star and its surface
temperature. The intensity of light emitted by three hypothetical stars is plotted against wavelength (compare
with Figure 4-2). The range of visible wavelengths is indicated. Where the peak of a star’s intensity curve lies
relative to the visible light band determines the apparent color of its visible light. The insets show stars of about
these surface temperatures. UV stands for ultraviolet, which extends to 10 nm. See Figure 3-4 for more on
wavelengths of the spectrum. (a: Hubble Heritage Team/AURA/STScI/NASA; left inset: Andrea Dupree/Harvard-
Smithsonian CFA, Ronald Gilliland/STScI, NASA and ESA; center inset: NSO/AURA/NSF; right inset: Till Credner, Allthesky.com)
FIGURE 11-7 A Hertzsprung-Russell Diagram On an H-R
diagram, the luminosities of stars are plotted against their
spectral types. Each dot on this graph represents a star whose
luminosity and spectral type have been determined. Some well known
stars are identified. The data points are grouped in just a
few regions of the diagram, revealing that luminosity and
spectral type are correlated: Main-sequence stars fall along the
red curve, giants are to the right, supergiants are on the top,
and white dwarfs are below the main sequence. The absolute
magnitudes and surface temperatures are listed at the right and
top of the graph, respectively. These are sometimes used on H-R
diagrams instead of luminosities and spectral types. (
FIGURE 11-7 A Hertzsprung-Russell Diagram On an H-R
diagram, the luminosities of stars are plotted against their
spectral types. Each dot on this graph represents a star whose
luminosity and spectral type have been determined. Some well known
stars are identified. The data points are grouped in just a
few regions of the diagram, revealing that luminosity and
spectral type are correlated: Main-sequence stars fall along the
red curve, giants are to the right, supergiants are on the top,
and white dwarfs are below the main sequence. The absolute
magnitudes and surface temperatures are listed at the right and
top of the graph, respectively. These are sometimes used on H-R
diagrams instead of luminosities and spectral types. (
(b) On this H-R diagram, each dot represents a main-sequence star. The number next to
each dot is the mass of that star in solar masses (M). As you
move up the main sequence from the lower right to the upper
left, the mass, luminosity, and surface temperature of mainsequence
stars all increase.
