Astronomy Midterm 2

Front 1

hotter stars are ___, while cooler stars are ___.

Back 1

blue/white; red/yellow

Front 2

astronomers can determine a star's surface temperature by:

Back 2

measuring the apparent brightness at different wavelengths, plotting a graph, and comparing it to theoretical blackbody curves to see which one matches the best

Front 3

on a Balmer thermometer, what do the dark lines mean?

Back 3

they show that there are chemical elements present in the star's atmosphere (called absorption lines)

Front 4

On a Balmer thermometer, different spectra mean different elements, which therefore means different ___.

Back 4

temperatures

Front 5

On a Balmer thermometer, when do spectral lines for hydrogen become visible?

Back 5

they occur when the electron starts in n = 2 and moves up to higher levels by absorbing photons

Front 6

Balmer lines of hydrogen are very ___.

Back 6

weak; thin, faint, hard to see.

Front 7

Stars of intermediate temperatures have ___ Balmer lines

Back 7

thick, bold, easy to see; strong

Front 8

Why are some lines stronger or easier to see than others on the Balmer thermometer?

Back 8

due to electronsa nd collisions between atoms, lines from other elements are strong or weak depending on the surface temp of a star. Cool stars = weak lines; hot stars = also weak lines; medium temp stars = STRONG lines b/c temperature just right (note: each chemical element has a different 'just right' middle point)

Front 9

Why do cool stars have weak lines on a Balmer thermometer?

Back 9

collisions between atoms are weak, electrons are in n = 1, and not enough collisional energy is passed to electrons to get them into n = 2, so the n = 2 level isn't populated by electrons.

Front 10

Why do hot stars have weak lines on a Balmer thermometer?

Back 10

the collisions between atoms are so strong that electrons are kicked right out, so few remain to get into n = 2 from n = 1

Front 11

Why do medium temperature stars create strong lines on a Balmer thermometer? How can different chemicals alter the admission lines?

Back 11

they are just the right temperature to get lots of electrons into n = 2 from n = 1, which create strong lines. Each chemical element has a different 'just right' middle point, so knowing what chemical element one is looking at is important to know how hot it is

Front 12

spectral types or spectral classes are a ____ way to estimate surface temperatures of stars

Back 12

more accurate

Front 13

In regards to spectral classes, the scales goes from __(hottest) to __(coolest)

Back 13

O - T

Front 14

What are the different types of spectral classes?

Back 14

O B A F G K M L T (hottest to coolest)

Front 15

What are the recently discovered 'cool objects' in the universe? (1995) Why aren't they classified as regular stars?

Back 15

brown dwarfs; they do not fuze hydrogen to helium in their cores, although some are hot enough to start the proton-proton chain, but only get to making 3He.

Front 16

4 characteristics of brown dwarfs

Back 16

- do not fuze hydrogen to helium in their cores

- they are objects between stars and jupiter-like planets

- cool (1300 - 2000 K)

- faint, small, hard to detect

Front 17

How can we measure a star's colour?

Back 17

viewing it through transmission filters, which let light of a certain wavelength through, but blocks all others (e.g. only allowing blue light = B filter, etc.)

Front 18

What is the UBV system in regards to analyzing star colours?

Back 18

u = ultraviolet, b = blue, v = visible (yellow)

Front 19

To calculate the colours of a star, you must ____.

Back 19

observe the star in 2 different filters, then combine the results (i.e. observe the star thru the B filter, say, & calculate a magnitude using only the blue photons. Then, observe thru a different filter (V, for ex.), and calculate a magnitude thru it.

- Note: adopt the letter of the filter as the symbol for the apparent magnitude, instead of using m.

- define colour index as (B - V) = magnitude in B minus the magnitude in V

- REMEMBER: the brighter the star, the LOWER the magnitude

Front 20

the smaller the colour index, the ___ & ___ the star

Back 20

bluer, hotter

Front 21

Suppose a star's magnitude thru the B filter is B = 11.1, and it's magnitude thru the V filter is V = 11.5. What is the colour index of the star?

Back 21

B - V: 11.1-11.5 = -0.4

Front 22

If B = 12.5 and V = 11.2, what is the colour index?

Back 22

B - V: 12.5 - 11.2 = 1.3

Front 23

If a star appears brighter in the red filter than the blue, then it is probably ___ in colour and ___ in temperature

Back 23

reddish; cool

Front 24

What's the problem with assessing a star's size through a telescope?

Back 24

most stars appear as points of light even thru telescopes

Front 25

We use information about luminosity and temperature to determine the ___ of stars. What is the equation?

Back 25

size; luminosity is proportional to radius squared x temperature to the fourth

Front 26

The purpose of an HR diagram is to ___.

Back 26

plot stars' characteristics

Front 27

How many stars are on the main sequence?

Back 27

80%

Front 28

What are the three main names of groups of stars that do not belong on the main sequence?

Back 28

giants, supergiants, and white dwarfs

Front 29

one characteristic of main sequence stars

Back 29

they are fusing hydrogen to helium in their cores (like the Sun)

Front 30

giants, supergiants, and dwarfs are all stars that are at different _____.

Back 30

stages of evolution

Front 31

giants (2)

Back 31

- evolved stars 10 - 100x the size of our sun

- brighter than our Sun

Front 32

A medium sized star on the main sequence will evolve into a ____, while a massive star on the main sequence will evolve into a ____.

Back 32

giant; supergiant

Front 33

dwarf

Back 33

a small star

Front 34

our Sun is classified as a ___ star

Back 34

dwarf (surprisingly?)

Front 35

red dwarf

Back 35

small, red, cool stars (spectral class M)

Front 36

white dwarfs

Back 36

very small (about the size of Earth)

very hot

Front 37

If two stars are the same temperature, does that also mean they are at the same stage of evolution?

Back 37

No; two stars can be the same temp. but at different stages of evolution

Front 38

luminosity

Back 38

the total energy output of a star per second

Front 39

larger stars have ____ dense atmospheres and ___ spectral lines

Back 39

less dense; narrower

Front 40

the more dense the gas of a star, the more ________, and the more pressure ____.

Back 40

collisions of atoms; broadening.

Front 41

stars of the same temperature but of different physical size must be at _____ stage(s) of evolution

Back 41

different

Front 42

the size of a star tells us how ___ it will be

Back 42

luminous

Front 43

What are the 6 different luminosity classes? What do they stand for?

Back 43

Ia= bright supergiant

Ib = supergiant

II = bright giant

III = giant

IV = subgiant

V = main sequence star

Front 44

in star identification, one should use both the ___ class and the ___ class

Back 44

spectral; luminosity. Ex: GV2 means the star is on the main sequence (V) and has a sepctral type of G2 (surface temp of 5800 K) = the sun

Front 45

what is spectroscopic parallax and how do you use it?

Back 45

- a new way to find distance

- uses the HR diagram to plot spectral type and luminosity

Front 46

how to use spectroscopic parallax

Back 46

1. get the star's spectral class (from pattern of absorption lines) and luminosity class (from width of lines)

2. measure the star's apparent magnitude (mv)

3. use spectral class and luminosity class to plot star on HR diagram, and read off Mv

4. calculate distance modulus (mv - Mv)

5. if you like, calculate the actual distance in pc.

Front 47

what is the furthest distance we can calculate distance using spectral types?

Back 47

10,000 pc

Front 48

the key to determining stellar masses is

Back 48

gravity

Front 49

Are star mass and star size two different things?

Back 49

YES

Front 50

mass

Back 50

how much 'stuff' a body contains

Front 51

size

Back 51

how big or small something is in physical dimensions

Front 52

binary stars

Back 52

two stars that orbit a common 'center of mass' like a balance point (i.e. think of two kids on a teeter totter)

Front 53

What happens if two binary stars are identical in mass? What if star A is more massive than star B?

Back 53

same mass: they orbit the same distance away from the center of mass. different: the more massive star orbits closer to the center of mass, and the less massive star orbits further away from the c. of m.

Front 54

what are the 3 different binary star types?

Back 54

visual binary, spectroscopic binary, and eclipsing binaries

Front 55

visual binary

Back 55

can resolve both stars through a telescope

Front 56

spectroscopic binary

Back 56

stars are so close together that through a scope, you only see one point of light

Front 57

eclipsing binary

Back 57

probably the most useful. The stars are still close so we only see one point of light through the scope, but the angle of the orbit relative to Earth is such that periodically, the stars pass in front of each other, blocking some of the light (so we see the light dimming periodically)

- plot a 'light curve:' the brightness of the whole system vs. time

Front 58

most stars are ___ (4)

Back 58

- as hot as the Sun or cooler

- as luminous as the Sun or cooler

- the same size as the sun or smaller

- as massive as the sun or less

Front 59

intrinsically luminous stars are ___, but easy to see even though they may be far away

Back 59

rare

Front 60

the brightest stars we see tend to be the most ___, not the most ____, while the nearest stars in space tend to be very ____, with only a few of them visible to the unaided eye

Back 60

luminous; nearby. faint

Front 61

material between stars is called _____, and all of it is called the _____.

Back 61

interstellar matter; interstellar medium

Front 62

what makes up the interstellar medium?

Back 62

2 parts: gas and dust

Front 63

space is NOT a vacuum because of the existence of ____

Back 63

interstellar medium

Front 64

99% of interstellar medium is ____; the remaining 1% is ___.

Back 64

gas atoms and molecules (mostly H and a bit of He), predominantly concentrated in nebulae. Solid frozen grain/dust that may be soot-like or sand-like

Front 65

nebula

Back 65

latin for 'cloud;' any cloud of gas and/or dust in space

Front 66

hydrogen in ISM may be ___, ____, or ___.

Back 66

ionized (HII), neutral (HI), part of a molecule (H2)

Front 67

HI

Back 67

"H one." Used for neutral hydrogen gas; not ionized

Front 68

HII

Back 68

"H two." Ionized hydrogen; hydrogen that's lost its electron

Front 69

H2

Back 69

"H two." Two hydrogen atoms stuck together

Front 70

soot-like dust

Back 70

lots of carbon; dark (soot, charcoal)

Front 71

sand-like dust

Back 71

silicates (silicone & oxygen). Like sand on Earth; light

Front 72

the typical size of dust particles in space are like the particles found in ____, not like the dust inside a house

Back 72

smoke (10s & 100s of nanometers big; comparable in size to the wavelength of blue light [400nm])

Front 73

since most of the gas in nebulae is ___, the HII region glows ___

Back 73

hyddrogen; red (strong red hydrogen line in spectrum)

Front 74

what are the 3 different kinds of nebulae?

Back 74

HII region (emission nebulae), reflection nebulae, and dark nebulae

Front 75

HII region (emission nebulae)

Back 75

- hot gas around or near a hot star

- the hot star's UV radiation ionizes the hydrogen gas around it

- nuclei recapture the electrons into higher energy levels

- electrons jump down those levels, and can produce lines (in spectrum) by emitting photons

Front 76

reflection nebulae

Back 76

- thin dust scattering blue light from nearby stars, and light is scattered towards Earth

- since blue wavelengths are comparable to the size of dust grains, the blue light is scattered, so the cloud appears to glow blue

Front 77

dark nebulae

Back 77

denser clouds of dust that block the light from the stars behind it

Front 78

the density of thin gas in nebulae is typically ____, as opposed to the air we breathe, which is ____. This means thin gas is extremely thin.

Back 78

one atom per cubic cm; 10000000000000000000(1x10 to 19) atoms per cubic cm

Front 79

If interstellar matter is so thin, how can it still make an impact in what we see?

Back 79

a cumulative effect (e.g. a foggy day)

Front 80

reddening

Back 80

more red light than blue light will pass through a particular ISM cloud, so cloud appears more red and skews the B-V colour index

Front 81

extinction

Back 81

the dimming of star light as it passes through thin dust (dust scatters away the blue photons so we record fewer photons from the star than we would if the dust wasn't there)

Front 82

cool clouds

Back 82

too cool (maybe 100 K) for there to be any high energy photons to kick out any electrons to show up in lines

Front 83

HI clouds

Back 83

neutral hydrogen gas, no lines in visible spectrum

Front 84

what are some properties of protons and electrons?

Back 84

mass, electric charge, spin (like a top; can spin clockwise or counterclockwise)

Front 85

In HI clouds, what happens when both the proton and electron spin the same way? Or in opposite ways?

Back 85

- when both proton and electron spin in the same direction, the energy is a bit higher than if one is spinning the other way

- if electron flips its spin so its opposite than that of the proton, the energy of the system decreases a bit, and a low energy, long-wavelength photon is emitted. In this, lambda = 21cm, which is in the radio part of the EM spectrum, so we find out they exist by detecting radiation.

Front 86

molecular clouds

Back 86

- dense clouds of gas and dust (but still thinner than air)

- giant molecular clouds are the birthplaces of stars

- known molecules: hydrogen, ammonia, carbon monoxide, water, formaldehyde, acetylene, alcohols, acetic acid, etc. Over 120 molecules now known

Front 87

giant molecular clouds spread over ____ pc across

Back 87

50

Front 88

in molecular clouds, the gas is ____; the cloud may develop a ____

Back 88

cool and thin (10 - 20 K = freezing); dense core

Front 89

triggers

Back 89

start the clumping process of molecular clouds. Various triggers put pressure on the cloud and cause it to clump up. Examples include shock waves and cloud collisions.

Front 90

how do shock waves cause clumping in giant molecular clouds?

Back 90

causes compression and clumping of the cloud material; the clumps them collapse and form stars. E.g. supernova explosions; stellar wind from massive star nearby; spiral arms of our galaxy

Front 91

how do cloud collisions cause clumping in giant molecular clouds?

Back 91

clouds are large and can interact w/ each other. The pressure of these interactions causes clumping

Front 92

free-fall collapse

Back 92

when atoms can fall into the core concentration in the center as if no material is around them at all

Front 93

Explain heating by contraction and how it creates protostars (4)

Back 93

- starting with giant molecular clouds where atoms free-fall collapse into the center of concentration

- the heat generated by collisions between the atoms can escape easily at first (b/c the cloud is very thin and big)

- core becomes opaque, dense, warmer than it was (still only about 100 K)

- as cloud (dense core) collapses, the heat gets trapped by the material, provides an outward pressure, and the contraction slows down

- central concentration of cloud now called a protostar, and it's shrouded in a cocoon of the leftover cloud still slowly collapsing in. The protostar itself is also still contracting

Front 94

protostar

Back 94

shines with energy released by contraction. Not yet hot enough to begin fusion, so it cannot be classified as a star yet

Front 95

why is the Orion nebula an 'excellent laboratory' for us to study on Earth? (2)

Back 95

- its relatively closeby (< 1500 LY) so features are resolved via telescope

- star formation began as a wave several million years ago, so we can see several stages of star formation in one place

Front 96

stellar wind

Back 96

stream of charged particles (e.g. protons and electrons) blowing off the 'surface' of a star

Front 97

star formation pillar

Back 97

signpost of triggered star formation

Front 98

birthline

Back 98

where protostars become visible

Front 99

When do protostars begin fusion?

Back 99

- some never begin fusion

- the timescale for collapse depends on the mass of the star

Front 100

evolutionary track

Back 100

line on HR diagram showing how a star's (or protostar's) luminosity and temperature changes w/ time as it evolves

Front 101

young stellar object (YSO) [2]

Back 101

- once protostar is revealed, its name changes to YSO. Still not fusing yet.

- YSOs often surrounded by protostellar disks; jets may be apparent

Front 102

When does a YSO become a star?

Back 102

YSO continues to collapse and get hotter. When the central temperature gets high enough, H -> He fusion begins, and it is a star on the main sequence

Front 103

disks

Back 103

formed from leftover material around protostars; material collapses down to form a disk around the protostar

Front 104

an upper limit to stars' masses exist because of (2)

Back 104

1. mass loss due to stellar wind: massive stars are very hot and have strong winds that can blow away much of the material

2. cloud fragmentation: You would need a massive cloud to remain intact and form one massive star, but we don't see this; they usually fragment up to form multiple stellar clouds

Front 105

hydrostatic equilibrium

Back 105

when a star is on the main sequence fusing H -> He, the pull of gravity towards the center is balanced by the outward gas pressure due to energy flow outwards and the temperature and density of the gas

Front 106

stars at the lower end of the main sequence are ___ to study, but are very numerous. ex: ____

Back 106

faint and difficult; brown dwarfs.

Front 107

a star begins its life on _____, and is _____.

Back 107

the zero-age main sequence (ZAMS); stable (mostly)

Front 108

As a star fuses H -> He in the core.. (4)

Back 108

- the # of nuclei gradually decreases, so core shrinks slightly

- temperature and density of core increases, which means increased energy generation

- more energy = more gas pressure outwards, which overpowers gravity in the outer layers of the star, so the outer layers expand a bit

- the star therefore appears a bit cooler but more luminous

Front 109

a star's lifetime on the main sequence depends on its ____. Regardless of mass, however, _____ of its life will be spent on the main sequence

Back 109

mass; 90%

Front 110

what does an evolutionary track do?

Back 110

follows a single star through changes in temperature and luminosity as it ages

Front 111

once hydrogen fuel is exhausted in a star's core, then no more ___ is generated; the core then mostly consists of ____. This is the beginning stage of a main sequence star becoming a ____.

Back 111

energy; helium; giant

Front 112

Once the core of a star stops fusing H to He and consists predominantly of He, the core ____, which produces enough ___ to ____. This causes the shell around the core to begin ____.

Back 112

collapses; energy; heat the shell of hydrogen around the core; fusing H to He (called the turn-off point on the HR diagram)

Front 113

After a main sequence star's outer shell begins fusing H to He (after its core stops and is predominantly He), ____ then causes the core to contract further

Back 113

gravity

Front 114

burning in a star = ___

Back 114

fusing

Front 115

Once a main sequence star's outer-core shell starts fusion/burning, this new fusion energy causes the star to ____, and the outer layers to ____. This causes the star to appear _____, and is finally considered a ____.

Back 115

swell up; cool. brighter, redder; giant

Front 116

the mass of a star on the main sequence dictates whether it will become a ___ or ___.

Back 116

giant; supergiant

Front 117

Panli Exclusion Principle

Back 117

no two electrons in any level can have the same properties

Front 118

degenerate matter

Back 118

a collection of free, noninteracting particles; gas is said to be 'degenerate'

Front 119

Once a giant's temperature reaches 100 million K, the triple alpha process begins. What is this?

Back 119

- an alpha particle is a helium nucleus, so we have 3 helium nuclei fusing to form one carbon nucleus

- This He to C fusion happens in the core via a series of reactions called the triple alpha process

Front 120

As a giant ages, its core fuses __ to ___ first, then __ to ___, and then becomes a core of __ and __in its final stage

Back 120

H -> He; He -> C; C & O2

Front 121

nucleosynthesis

Back 121

the cosmic formation of atoms more complex than the hydrogen atom (e.g. the creation of carbon by a star's core)