Lecture 12 - Surveying Stars

Question 1

what is luminosity and how does it relate to energy?

Answer 1

luminosity = amount of power a star radiates, an absolute measure, regardless of distance

energy per second (watts)

Question 2

what is absolute brightness and how does it relate to energy?

Answer 2

amount of starlight that we see

energy per second per square meter, i.e. watts per square meter

Question 3

how do the luminosity and brightness of a star change as it moves further away?

Answer 3

always puts out the same amount of luminosity but will look dimmer if it is further away

Question 4

if Alpha Centauri and the sun have the same luminosity, which appears brighter?

Answer 4

the sun because it is closer

Question 5

what is the inverse square law for light? (description, equation, and units)

Answer 5

the apparent brightness decreases with the square of its distance because the same amount of light much pass through each larger sphere surrounding the star

apparent brightness = (luminosity)/(4pi*distance^2)

units: watts per square meter

Question 6

what is the 4pi*distance^2 part of the equation representing?

Answer 6

distance = distance from star

4pi*radius^2 = area of sphere

Question 7

based on the inverse square law for light, what would happen if we view the sun from 2x Earth’s distance? 10x?

Answer 7

2x: would appear dimmer by a factor of 2^2

10x: would appear dimmer by a factor of 10^2

Question 8

at a distance of 2AU, what happens to traveling light compared to 1AU?

Answer 8

travels through 4x more area

Question 9

at a distance of 3AU, what happens to traveling light compared to 1AU?

Answer 9

travels through 9x more area

Question 10

X-ray telescopes only detect:

Answer 10

X-ray luminosity and brightness

Question 11

when can we detect total luminosity/apparent brightness?

Answer 11

if we detect photons across the whole electromagnetic spectrum

Question 12

why is the inverse square law for light not necessarily realistic?

Answer 12

assumes light follows an uninterrupted path to Earth, but in reality light passes thru dust and gas clouds that absorb or scatter the light

Question 13

what do we use to measure distance? explain what this is

Answer 13

we use STELLAR PARALLAX

the annual shift in a star’s apparent position due to Earth’s motion around the sun

Question 14

why do stars appear to shift?

Answer 14

because we observe a star from 2 different points of earth’s orbit, i.e. diff times of year

Question 15

describe how we measure a star’s distance using parallax and the 2 equations we can use

Answer 15

measure parallax angle (angle of star relative to earth and sun)

d (in parsecs) = 1/(p (in arcseconds))
d (in lyr) = 3.26 (1/(p (in arcseconds)))

Question 16

how does parallax angle change if a star is further away?

Answer 16

if star is further away, parallax angle is smaller

Question 17

why could the Greeks never measure parallax?

Answer 17

most stars have parallax angle < 1arcsecond but our eyes can only resolve 1 arcminute

Question 18

1 parsec = ?

Answer 18

1 parsec = 3.26 light-years

Question 19

2 reasons why parallax measurements are helpful?

Answer 19

1. only way to measure distances without making assumptions about the nature of stars
2. lets us calculate luminosity

Question 20

what is the range of luminosities of stars? are most stars dim or bright?

Answer 20

10^-4 to 10^6 L_sun

dim stars are most common

sun has mid luminosity but brighter than most stars

Question 21

describe stellar temperatures

Answer 21

every object emits THERMAL RADIATION with a spectrum that depends on its temperature

with a fixed size, an object will be MORE LUMINOUS as TEMP RISES

Question 22

why are hotter objects more luminous?

Answer 22

emits more light per unit area at all frequencies and emit photons with higher average energy

Question 23

what part of the star does its temperature give information about?

Answer 23

SURFACE

Question 24

what are the 2 properties of star that indicate a star’s temperature?

Answer 24

1. colour
2. level of ionization

Question 25

what does the colour tell us about a star? what do blue stars represent? what do red stars represent? why is the sun yellow?

Answer 25

colour indicates its temeperature

hottest stars = 50,000K = more blue
coolest stars = 3000K = more red
sun = 5800K = middle of visible spectrum, yellow

Question 26

what does the level of ionization tell us about a star? what does it mean when there is more ionization?

Answer 26

level of ionization indicates its temperature

Question 27

how can we detect the ionization in a star?

Answer 27

using absorption lines in its spectrum

if spectral lines of highly ionized elements: high temp
if spectral lines of molecules: low temp

Question 28

why are absorption lines more accurate than colour?

Answer 28

interstellar dust can affect the apparent colour

Question 29

how are stars classified based on spectra?

Answer 29

spectral type: (hottest)OBAFGKM(coldest)

Question 30

describe the H lines in O group

Answer 30

weak H lines because they have high temp that causes all H to be ionized

Question 31

describe the lines in M group

Answer 31

strong molecular lines in M group because they are cool enough to have stable molecules

Question 32

describe the amount of elements you will see in spectra at lower temp

Answer 32

will see MORE elements at LOWER temp

Question 33

how can we measure stellar masses and what is the caveat?

Answer 33

use KEPLERS 3RD LAW

caveat: only works when 1 object is orbiting another, therefore only works for binary star systems

Question 34

why does keplers law only work for orbiting objects?

Answer 34

calculation requires orbital period and average orbital distance of the orbiting object

Question 35

what is an example of a binary star system

Answer 35

Mizar star

Question 36

what does the orbit of a binary star system depend on?

Answer 36

strength of gravity

Question 37

what are the 3 types of binary systems?

Answer 37

1. visual binary
2. spectroscopic binary
3. eclipsing binary

Question 38

what is a visual binary?

Answer 38

we can see each star separately

Question 39

is it possible that a visual binary appears to be 1 star?

Answer 39

yes, we may be able to see 1 star slowly shifting and a dim companion

Question 40

what is a spectroscopic binary? what allows us to detect them?

Answer 40

don’t visually see the stars differently but can identify DOPPLER SHIFT in its spectral lines

when a star orbits another, it is moving away and towards us in its orbit so the spectral lines show alternating blueshifts and redshifts

Question 41

what are the 2 types of spectroscopic binary?

Answer 41

1. double-lined spectroscopic binary: 2 sets of lines shifting back and forth, 1 set from each star
2. single-lined spectroscopic binary: only 1 set of lines shifting back and forth because 1 star is too dim to be detected

Question 42

what is an eclipsing binary? what are the 3 ways we can them?

Answer 42

stars orbiting in the plane of our line of sight

1. when neither star is eclipsed, we see the combined light of both stars
2. when smaller one in front, we see all the light of the front one and some of the light of the back one
3. when larger one in front, we only see the light of the front one

Question 43

what are the 2 equations we use to measure mass?

Answer 43

Question 44

what is the range of star mass?

Answer 44

0.08 M_sun to 100 M_sun