Lecture 12 - Surveying Stars Flashcards
what is luminosity and how does it relate to energy?
luminosity = amount of power a star radiates, an absolute measure, regardless of distance
energy per second (watts)
what is absolute brightness and how does it relate to energy?
amount of starlight that we see
energy per second per square meter, i.e. watts per square meter
how do the luminosity and brightness of a star change as it moves further away?
always puts out the same amount of luminosity but will look dimmer if it is further away
if Alpha Centauri and the sun have the same luminosity, which appears brighter?
the sun because it is closer
what is the inverse square law for light? (description, equation, and units)
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
what is the 4pi*distance^2 part of the equation representing?
distance = distance from star
4pi*radius^2 = area of sphere
based on the inverse square law for light, what would happen if we view the sun from 2x Earth’s distance? 10x?
2x: would appear dimmer by a factor of 2^2
10x: would appear dimmer by a factor of 10^2
at a distance of 2AU, what happens to traveling light compared to 1AU?
travels through 4x more area
at a distance of 3AU, what happens to traveling light compared to 1AU?
travels through 9x more area
X-ray telescopes only detect:
X-ray luminosity and brightness
when can we detect total luminosity/apparent brightness?
if we detect photons across the whole electromagnetic spectrum
why is the inverse square law for light not necessarily realistic?
assumes light follows an uninterrupted path to Earth, but in reality light passes thru dust and gas clouds that absorb or scatter the light
what do we use to measure distance? explain what this is
we use STELLAR PARALLAX
the annual shift in a star’s apparent position due to Earth’s motion around the sun
why do stars appear to shift?
because we observe a star from 2 different points of earth’s orbit, i.e. diff times of year
describe how we measure a star’s distance using parallax and the 2 equations we can use
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)))
how does parallax angle change if a star is further away?
if star is further away, parallax angle is smaller
why could the Greeks never measure parallax?
most stars have parallax angle < 1arcsecond but our eyes can only resolve 1 arcminute
1 parsec = ?
1 parsec = 3.26 light-years
2 reasons why parallax measurements are helpful?
- only way to measure distances without making assumptions about the nature of stars
- lets us calculate luminosity
what is the range of luminosities of stars? are most stars dim or bright?
10^-4 to 10^6 L_sun
dim stars are most common
sun has mid luminosity but brighter than most stars
describe stellar temperatures
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
why are hotter objects more luminous?
emits more light per unit area at all frequencies and emit photons with higher average energy
what part of the star does its temperature give information about?
SURFACE
what are the 2 properties of star that indicate a star’s temperature?
- colour
- level of ionization
what does the colour tell us about a star? what do blue stars represent? what do red stars represent? why is the sun yellow?
colour indicates its temeperature
hottest stars = 50,000K = more blue
coolest stars = 3000K = more red
sun = 5800K = middle of visible spectrum, yellow
what does the level of ionization tell us about a star? what does it mean when there is more ionization?
level of ionization indicates its temperature
how can we detect the ionization in a star?
using absorption lines in its spectrum
if spectral lines of highly ionized elements: high temp
if spectral lines of molecules: low temp
why are absorption lines more accurate than colour?
interstellar dust can affect the apparent colour
how are stars classified based on spectra?
spectral type: (hottest)OBAFGKM(coldest)
describe the H lines in O group
weak H lines because they have high temp that causes all H to be ionized
describe the lines in M group
strong molecular lines in M group because they are cool enough to have stable molecules
describe the amount of elements you will see in spectra at lower temp
will see MORE elements at LOWER temp
how can we measure stellar masses and what is the caveat?
use KEPLERS 3RD LAW
caveat: only works when 1 object is orbiting another, therefore only works for binary star systems
why does keplers law only work for orbiting objects?
calculation requires orbital period and average orbital distance of the orbiting object
what is an example of a binary star system
Mizar star
what does the orbit of a binary star system depend on?
strength of gravity
what are the 3 types of binary systems?
- visual binary
- spectroscopic binary
- eclipsing binary
what is a visual binary?
we can see each star separately
is it possible that a visual binary appears to be 1 star?
yes, we may be able to see 1 star slowly shifting and a dim companion
what is a spectroscopic binary? what allows us to detect them?
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
what are the 2 types of spectroscopic binary?
- double-lined spectroscopic binary: 2 sets of lines shifting back and forth, 1 set from each star
- single-lined spectroscopic binary: only 1 set of lines shifting back and forth because 1 star is too dim to be detected
what is an eclipsing binary? what are the 3 ways we can them?
stars orbiting in the plane of our line of sight
- when neither star is eclipsed, we see the combined light of both stars
- when smaller one in front, we see all the light of the front one and some of the light of the back one
- when larger one in front, we only see the light of the front one
what are the 2 equations we use to measure mass?
what is the range of star mass?
0.08 M_sun to 100 M_sun
