Ch. 14.2 (Stars)

Question 1

Star used as standard reference point?

Answer 1

• The Sun is used as a reference for the understanding all the other stars.
• Sun ordinary star with average brightness.
• Like all other stars, the Sun is a massive, dense ball of gases with a surface heated to incandescence by energy released from fusion reactions deep within.

Question 2

Nebula (nebulae)?

Answer 2

• A diffuse mass of interstellar clouds of hydrogen gas or dust.
• Consists of random, swirling atoms of gases that have little gravitational attraction for one another because they have little mass.
• Complex motions of stars produce a shock wave that causes local compressions, forcing particles to move closer together and collide.
• Their mutual gravitational attraction then begins to pull them together into a cluster.

Question 3

Protostar?

Answer 3

• An accumulation of gases that will become a star.
• Theoretical calculations indicate that prior to the formation of a sun-like star, a collection of approximately 1 × 10^57 atoms, within a distance of 3 trillion km (about 1.9 trillion mi), are needed to form a protostar.

Question 4

Core characteristics?

Answer 4

• The core is a dense, very hot region where nuclear fusion reactions release gamma and X-ray radiation.
• The density of the core is about 12 times that of solid lead.
• Because of the plasma conditions, however, the core remains in a gaseous state even at this density.

Question 5

Radiation zone characteristics?

Answer 5

• The radiation zone is less dense than the core, having a density about the same as that of water.
• Energy in the form of gamma and X rays from the core is absorbed and reemitted by collisions with atoms in this zone.
• The radiation slowly diffuses outward because of the countless collisions over a distance comparable to the distance between Earth and the Moon.
• It could take millions of years before this radiation finally escapes the radiation zone.

Question 6

Convection zone characteristics?

Answer 6

• The convection zone begins about seven-tenths of the way to the surface, where the density of the gases is about 1 percent of the density of water.
• Gases at the bottom of this zone are heated by radiation from the radiation zone below, expand from the heating, and rise to the surface by convection.
• At the surface, the gases emit energy in the form of visible light, ultraviolet radiation, and infrared radiation, which moves out into space. As they lose energy, the gases contract in volume and sink back to the radiation zone to become heated again, continuously carrying energy from the radiation zone to the surface in convection cells.
• The surface is continuously heated by the convection cells as it gives off energy to space, maintaining a temperature of about 5,800 K (about 5,500°C).

Question 7

Life spans of different stars?

Answer 7

• More massive stars generate higher temperatures in the core because they have a greater gravitational contraction from their greater masses.
• Higher temperatures mean increased kinetic energy, which results in increased numbers of collisions between hydrogen nuclei with the end result being an increased number of fusion reactions. Thus, a more massive star uses up its hydrogen more rapidly than a less massive star.
• On the other hand, stars that are less massive than the Sun use their hydrogen at a slower rate, so they have longer life spans.
• The life spans of the stars range from a few million years for large, massive stars, to 10 billion years for average stars like the Sun, to trillions of years for small, less massive stars.

Question 8

Factors of a star’s brightness?

Answer 8

• The amount of light produced by the stars.
• The size of each star.
• The distance to a particular star.

Question 9

Apparent magnitude?

Answer 9

• A classification scheme for different levels of brightness that you see.
• Brightness values range from 1 to 6, with the number 1 assigned to the brightest star and the number 6 assigned to the faintest star that could be seen.
• Stars assigned the number 1 came to be known as first-magnitude stars, those a little dimmer as second-magnitude stars, and so on to the faintest stars visible, the sixth-magnitude stars.
• The apparent magnitude of a star depends on how far away stars are in addition to differences in the stars themselves. Stars at a farther distance will appear fainter, and those closer will appear brighter, just as any other source of light does.

Question 10

Apparent magnitude scale

Answer 10

• Typical ranges from -25 to +20
• Sun @ -25
• Sirius @ 0
• Naked eye limit with large telescope @ +20

Question 11

Different brightness magnitudes of stars

Answer 11

• A first-magnitude star is defined as one that is 100 times brighter than a sixth-magnitude star, with five uniform multiples of decreasing brightness on a scale from the first magnitude to the sixth magnitude.
• Each numerical increase in magnitude represents a change in brightness that is 2.51 times brighter than the previous magnitude. This means that first-magnitude star is 2.51 times brighter than a second-magnitude star, and 6.3 times brighter than a third-magnitude star.
• Brightness equation: 2.51^x=b2/b1; where x is the difference in magnitude between the two objects, and b2/b1 is the ratio of brightness between the two stars.

Question 12

Absolute magnitude?

Answer 12

• A classification scheme to compensate for the distance differences to stars, calculations of the brightness that stars would appear to have if they were all at a defined, standard distance (32.6 light-years).
• An expression of luminosity.

Question 13

Luminosity?

Answer 13

• The total amount of energy radiated into space each second from the surface of a star.
• The Sun, for example, radiates 4 × 1026 joules per second from its surface.
• The luminosity of stars is often compared to the Sun’s luminosity, with the Sun considered to have a luminosity of 1 unit.
• When this is done, the luminosity of the stars ranges from a low of 10^−6 sun units for the dimmest stars up to a high of 10^5 sun units. Thus, the Sun is somewhere in the middle of the range of star luminosity.

Question 14

Star temperature and star color?

Answer 14

• The colors of the various stars are a result of the temperatures of the stars.
• This color difference is understood to be a result of the relationship that exists between the color and the temperature of an incandescent object.
• A cooler star are seen as reddish and comparatively hotter stars as bluish white. Stars with in-between temperatures, such as the Sun, appear to have a yellowish color.

Question 15

Spectral types and temperatures

Answer 15

(Type, color, temperature in K.)

O - Bluish 	  30,000–80,000
B - Bluish 	  10,000–30,000
A - Bluish           7,500–10,000
F - White   	   6,000–7,500
G - Yellow 	   5,000–6,000
K - Orange-red  3 ,500–5,000
M - Reddish 	   2,000–3,500

Question 16

Wien’s displacement law?

Answer 16

• Laboratory experiments indicate that the temperature of bodies of gases, including stars, can be determined from the peak wavelength of the light the object emits.
• The temperature (in kelvins) is inversely proportional to the peak wavelength, in a relationship known as Wien’s displacement law.
• The relationship between peak wavelength and temperature is:

T=2.897 × 10 K⋅angstroms/λpeak

Question 17

H-R diagram?

Answer 17

• The Hertzsprung-Russell diagram, or the H-R diagram for short.
• The diagram is a plot with temperature indicated by spectral types and the true brightness indicated by absolute magnitude.
• The diagram plots temperature by spectral types sequenced O through M, so the temperature decreases from left to right.
• The hottest, brightest stars are thus located at the top left of the diagram, and the coolest, faintest stars are located at the bottom right.
• Each dot is a data point representing the surface temperature and brightness of a particular star. The Sun, for example, is a type G star with an absolute magnitude of about +5, which places the data point for the Sun almost in the center of the diagram. This means that the Sun is an ordinary, average star with respect to both surface temperature and true brightness.

Question 18

Main sequence stars?

Answer 18

• Most of the stars plotted on an H-R diagram fall in or close to a narrow band that runs from the top left to the lower right. This band is made up of main sequence stars.
• Stars along the main sequence band are normal, mature stars that are using their nuclear fuel at a steady rate.
• Those stars on the upper left of the main sequence are the brightest, bluest, and most massive stars on the sequence. Those at the lower right are the faintest, reddest, and least massive of the stars on the main sequence.
• In general, most of the main sequence stars have masses that fall between a range from 10 times greater than the mass of the Sun (upper left) to one-tenth the mass of the Sun (lower right).
• The extremes, or ends, of the main sequence range from about 60 times more massive than the Sun to one-tenth of the Sun’s mass.
• It is the mass of a main sequence star that determines its brightness, its temperature, and its location on the H-R diagram. High-mass stars on the main sequence are brighter and hotter and have shorter lives than low-mass stars. These relationships do not apply to the other types of stars in the H-R diagram.