AST 104 (jeffery ial) EXAM 2 pt2 (Questions I missed a lot during practice)

The surface (i.e., photosphere) temperature of an ordinary star can be determined from:

the shape of its approximately BLACKBODY spectrum (particularly the location of the peak) and/or an analysis of its ABSORPTION line spectrum.

Electromagnetic radiation (EMR) is:

a WAVE PHENOMENON... NOT EXCITATIONS OF A MEDIUM... IN VACUUM... the speed of light is always LOWER.

The surface (i.e., photosphere) temperature of an ordinary star can be determined by:

identifying its spectral type.

A true blackbody absorbs all the electromagnetic radiation that hits it (i.e., it does not reflect any electromagnetic radiation) and has a uniform temperature. Let us treat the Earth as blackbody, except that it reflects that fraction of light that it actually does reflect. The light gathering surface area of the Earth is πR2 ⊕ , where π ≈ 3.1416 is pi, a pure number, and R⊕ is the Earth radius. The total light energy gathered per unit time by the Earth is thus f(1 − a)πR2 ⊕ , (1) where f = 1367.6 watts per square meter is the mean solar constant and (1 − a) = 0.694 is a factor accounting for the reflection of electromagnetic radiation from the Earth (Wikipedia: Earth: Bond albedo). As a blackbody (except for the reflection correction), the Earth radiates a total energy per unit time of AσT 4 , (2) where A = 4πR2 ⊕ 5 is the surface area of the Earth and σT 4 is the Stefan-Boltzmann law (i.e., the energy radiated per unit area per unit time by a blackbody). The constant σ = 5.670373 × 10−8 in mks units. Since the Earth is neither a net energy gainer or loser (at least not to an extent important for this problem), expression (1) must equal expression (2) to maintain a constant thermal energy content on Earth. Equating the expressions, we obtain: f(1 − a) = 4σT 4 or T = f(1 − a) 4σ 1/4 = 254 K . This temperature is called the blackbody or effective temperature of the Earth.

At 254 K the Earth would be colder than the freezing point of water. The reason the Earth isn't this cold is because of the greenhouse HEATING effect.

The energy emitted as electromagnetic energy from main sequence stars is supplied by the:

nuclear burning of hydrogen to helium

They are the most luminous stars (i.e., luminosities of order 106 L⊙) and put in luminosity class 0. They are called:

hypergiants.

The solar wind is a stream of particles that moves approximately along radial paths outward from the Sun: inward is the negative direction and positive is the outward direction. The solar wind near the Earth is typically moving at a radial velocity of about:

400 to 500 km/s.

"Let's play Jeopardy! For $100, the answer is: Each stellar spectral types is divided into these subtypes."

What are (BLANK), Alex?

0, 1, 2, . . . , 9

A common reason why some astrophysical systems are described as poorly understood is that these systems involve three-dimensional hydrodynamic effects (e.g., convection).

Three-dimensional hydrodynamics can OFTEN be understood qualitatively and this SOMETIMES allows us to predict three-dimensional hydrodynamical phenomena. Accurate computations of three-dimensional hydrodynamic effects are also possible in some cases.

"Let's play Jeopardy! For $100, the answer is: These isotopes of hydrogen have 1 and 2 neutrons, respectively."

What are (BLANK), Alex?

the deuteron (D or 2 1H) and triton (T or 3 1H)

The contraction of a protostar is halted eventually by:

the heat generated by the turning on of nuclear burning which INCREASES the gas pressure inside the protostar.

For a main sequence star, the energy radiated away as electromagnetic radiation is almost exactly compensated by:

energy produced by nuclear burning in the deep interior.

In stellar hydrogen fusion to helium, the rest mass energy of the products is (BLANK) less than that of the reactants. The missing rest mass energy went mostly into (BLANK).

0 %; chemical binding energy

If all the stellar parallaxes (i.e., parallax angles measured during a half revolution of the Sun) were INCREASING with time, this would mean that the stars were all:

moving closer.

Main sequence stars of low mass are mainly supported against collapse ( >∼ 90 % for M <∼ 8M⊙) by:

the ideal gas pressure of ions and electrons.

ecause gravity is a long-range, inverse-square-law force, significant gravitational interactions between two stars:

are relatively common.

In a FREE-FALL contraction of part of molecular cloud:

the part starts fall to toward a high density point because of gravitational attraction. Pressure forces are negligible in slowing the fall because it is a free-fall contraction.

The dense, cold component of the interstellar medium from which stars are believed to form is made of:

molecular clouds.

The diameter of the Sun is about:

109 Earth diameters.

Interstellar dust probably varies widely in composition, size scale, and structure. But there some ideas about typical dust that are generally accepted.

Although size scale probably varies widely, a typical dust grain may be of order 1 µm = 10−6 m in size, but it won't be perfectly spherical. There may be a core of NONVOLATILE material of order 0.05 µm consisting of silicates (silicon and oxygen compounds that make up most terrestrial rock), iron, or graphite. The GRAIN MANTLE may be mostly VOLATILE ICES: e.g., H2O (water ice), CO2 (carbon dioxide ice or dry ice), CH4, and NH3. The grain surface may have complex molecules forming tarry substances. Dust probably forms in STELLAR WINDS AND SUPERNOVA EJECTA. There relatively dense NONVOLATILES condense out forming the cores as the ejected gas cools. As the gas cools more, VOLATILES condense out on the cores.

A dim star is located at about 2 million astronomical units from Earth. Recall 1 AU = 1.496 × 10^11 m and 1 pc = 3.09 × 10^16 m. Approximately, what is the distance to the star in parsecs?

10 pc.

"Let's play Jeopardy! For $100, the answer is: These are loosely-bound, irregularly-shaped groups of stars consisting of order 100 to 1000 stars and having size scales of order 4 to 20 pc."

What are (BLANK), Alex?

open clusters

The approximate colors of the hydrogen Balmer lines Hα, Hβ, Hγ, and Hδ are, respectively:

red, blue-green, blue-violet, and violet.

"Let's play Jeopardy! For $100, the answer is: These objects appear on Hertzsprung-Russell diagrams and they are assigned a luminosity class VII."

What are (BLANK), Alex?

white dwarfs

The corona of the Sun is only visible to the naked eye:

during total solar eclipses.

The life history of our own star, the Sun, is known to us by:

direct observations of its current stage plus observations of other stars in all their stages and modeling.

"Let's play Jeopardy! For $100, the answer is: They are relatively thin, round objects consisting of gas and/or dust and/or particles: the material goes around some large astro-body in nearly circular orbits of varying radii in the same direction."

What are (BLANK), Alex?

disks

An object that forms in a star formation region with less than about 0.08 M⊙, but more than about 13 Jupiter masses (according to one school of thought), and which never burns ordinary hydrogen is called a:

white dwarf.

"Let's play Jeopardy! For $100, the answer is: These are structures of a few to a few hundred stars and span of order 10 to 100 pc. They are generally gravitationally unbound though gravitationally interacting."

What are (BLANK), Alex?

associations

These main sequence stars have masses in the range 0.08-0.4 M⊙. They have the lowest temperatures and densities in their cores of all main sequence stars and subsequently burn hydrogen to helium most slowly. Convection occurs throughout these stars and eventually they will be converted entirely into helium. They will never burn any other nuclear fuel and eventually must become helium white dwarfs. Their main sequence lifetimes are predicted by models to be hundreds of billions of years. According to our current cosmological theory the age of the universe is only about 14 billion years. Thus, none of these stars has ever left the main sequence. These stars are called:

red dwarfs.

According to one standard reference, the solar luminosity L⊙ = 3.846 × 1026 W http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html 2013 and the solar constant (i.e., the solar flux at the mean distance of the Earth) f = 1367.6 W/m2 . Stellar luminosity L and flux f are related by the inverse-square law f = L 4πd2 , where d is the distance from the center of the star to the location where f is measured. Solve for d analytically and then find mean Earth-Sun distance.

d = p L/(4πf) and d = 1.496 × 1011 m.

The hydrogen Balmer lines in main sequence stars:

are strongest at surface temperature of order 10, 000 K.