A13 - Exploring starlight

Question 1

Describe the difference between apparent and absolute magnitude, and link them together with an equation:

Answer 1

apparent - measured brightness from its actual distance

absolute - measured brightness if all objects were 10pc away

In both, negative values are brighter than positive ones

distance modulus formula

Question 2

Explain the difference in brightnesses when using magnitude system:

Answer 2

a difference in 5 magnitudes corresponds to a 100x change in brightness (meaning the difference between 2 magnitudes is 2.5x, or the 5th root of 100)

Question 3

What information can be obtained from a stellar spectrum?

Answer 3

-chemical composition (with spectral absorption/emission lines)
-temperature
-radial/recessional velocity (from red/blueshift)

Question 4

Describe the method of classifying stars by their temperature:

Answer 4

-Harvard classification (OBAFGKM), which is subdivided into smaller divisions of 0-9 (0 is hottest, 9 is coolest)

-hotter, blue stars are O
-cooler, red stars are M

Question 5

How can we present the information about stars on a diagram?

Answer 5

-the Hertzsprung-Russel (HR) diagram

-temperature and spectral class is the x-axis
-luminosity (log, Sun=1) and absolute magnitude is the y-axis

Question 6

Draw a labelled HR diagram:

Answer 6

key points
-main sequence
-Sun (m=4.8, G2)
-red/blue giants
-white dwarfs
-supergiants

Question 7

How do stars of a similar mass to the Sun evolve on the HR diagram?

Answer 7

Question 8

How do high mass stars evolve on the HR diagram?

Answer 8

Question 9

Why aren’t neutron stars on the HR diagram?

Answer 9

-they don’t have a spectral type
-not observed in visible wavelengths

Question 10

Describe how we measure distances to nearby stars:

Answer 10

heliocentric parallax
-measure star’s position relative to other distant background stars
-measure the same star’s position 6 months later and record its parallax angle
-use trig. and the distance from Earth to the Sun to work out the distance to the star

Question 11

Which angle is the parallax angle?

Answer 11

Question 12

How can you use a HR diagram to determine the distance to a star?

assuming it is a main sequence star

Answer 12

spectroscopic parallax
-determine spectral class/temperature from its spectral lines
-draw a line up from its corresponding temperature to the main sequence line, and draw another line horizontally to get its absolute magnitude
-measure apparent magnitude and use the distance-modulus formula to calculate how far it is

Contrary to the name, nothing about this method uses parallax

Question 13

Name 4 types of stars that have a variable light curve:

Answer 13

-short/long period
-eclipsing binary
-Cepheid variables
-novae/supernovae

Question 14

What are short and long period stars? Draw their light curve:

Answer 14

short - stars with a period of several days/weeks

long - giant stars with periods between a hundred and a thousand days

Question 15

What are eclipsing binary stars? Draw their light curve:

Answer 15

a binary star system where one star may pass in front of the other, causing a notable change in brightness

When the brighter star is hidden from view/blocked, the dip is larger

Question 16

What are Cepheid variables? Draw their light curve:

Answer 16

a star that pulses frequently

A steep increase, following by a shallow decrease

Question 17

How can a Cepheid variable be used to determine its distance?

Answer 17

-find period of star by looking at its light curve
-find its luminosity using the period-luminosity relationship
-use HR diagram to find its corresponding absolute magnitude
-use average apparent magnitude and plug into the distance-modulus formula to calculate the distance to the star

Question 18

Describe 2 types of clusters:

Answer 18

open - young groups of dozens of stars (eg M45, Pleiades) with no particular symmetry, found in the outer arms of the galaxy

globular - old groups of 100,000s of stars (eg M13, Hercules cluster) in a ball shape, found near to the galactic nucleus

Question 19

How can you determine the length of a sidereal day?

Answer 19

star trail photos
-take long exposure photo (of a known time) of the sky around Polaris
-measure angle of arc of one star trail with respect to Polaris (close enough to the NCP)

-use ratios to calculate how long it would take for the star to rotate 360°, ie a full sidereal day

Question 20

Describe how a digital camera records pictures:

Answer 20

-light from subject passes through lens and is refracted so it gets focused onto the CMOS/CCD sensor
-the sensor measures the light received
-image gets recorded as electrical signals which can be processed digitally

CMOS - complementary metal oxide semiconductor
CCD - charge-coupled device

Question 21

How can a stellar spectrum be obtained?

Answer 21

pass light from star through a spectrometer attached to the telescope

the old method uses a prism

Question 22

How does the Earth’s atmosphere affect astronomical observations?

Apart from seeing conditions

Answer 22

the atmosphere blocks almost all wavelengths of EM radiation apart from visible and radio waves

Question 23

Describe where the different types of telescopes must be placed:

Answer 23

-visible and radio can be placed on the ground (as they pass through the atmosphere)

-others must be above because the atmosphere blocks the rest

Question 24

How do radio telescopes work?

Answer 24

-large parabolic primary dish focuses the radio waves onto a secondary receiver
-secondary receiver converts the radio wave signals into electrical signals

Question 25

Why are radio telescopes made of metal?

Answer 25

metal reflects radio waves ## Footnote glass doesn't

Question 26

Why do radio telescopes have to be so large?

Answer 26

the wavelength is very large, so to maintain a better angular resolution the diameter must also be very large ## Footnote In the equation, ensure the wavelength and diameter have the same units

Question 27

How can multiple radio telescopes be used together?

Answer 27

*aperture synthesis system (array)* -uses interferometry to combine signals from dishes **that are spread out over a large distance**, and effectively produces a telescope with a diameter of the distance between the 2 furthest dishes

Question 28

What has been discovered through the use of radio telescopes?

Answer 28

-quasars -jets from black holes -protoplanetary disks -structure of Milky Way -CMBR

Question 29

Where are infrared telescopes located?

Answer 29

higher altitudes in dry locations (water vapour absorbs IR)

Question 30

What has infrared astronomy helped to discover?

Answer 30

-protostars (areas of heat behind dust clouds) -dust and molecular clouds (usually dark in visible wavelengths) -hotspots on the Moon

Question 31

What effect does Earth's atmosphere have on astronomical images from ground-based telescopes? ## Footnote Excluding certain wavelength being reflected

Answer 31

-clouds obscure view -atmosphere refracts light (and variably) so star position/clarity is less accurate

Question 32

What are the pros of using space telescopes?

Answer 32

-less atmospheric interference (eg twinkling/light pollution) -can observe at more wavelengths -no limitations to just observing at night

Question 33

Give some cons of using space telescopes:

Answer 33

-usually can't image in the direction of the Sun/Moon -very expensive to build and maintain -hard to position

Question 34

What have gamma telescopes helped us to observe?

Answer 34

gamma ray bursts

Question 35

What have x-ray telescopes helped us to observe?

Answer 35

black hole accretion disks

Question 36

What have ultraviolet telescopes helped us to observe?

Answer 36

the structure of the corona and chromosphere of stars (including the Sun)

Question 37

Describe how a telescope alters the appearance of stars and double stars:

Answer 37

*single stars* - makes them sharper points of light *double/binary stars* - distinguishes the single point of light into 2 ## Footnote Double stars are 2 stars that appear close together, and are usually just binary star systems (not always though, eg optical/visual doubles)

Question 38

How does a telescope alter the appearance of open and globular clusters?

Answer 38

*open* - usually a few faint stars but a telescope causes numerous individual stars to be seen *globular* - usually a fuzzy blur but a telescope resolves them into many

Question 39

How does a telescope alter the appearance of nebulae and galaxies?

Answer 39

makes shape more detailed and colour more obvious

Question 40

What is a binary star system?

Answer 40

2 stars that are **gravitationally** bound to each other, and orbit a common centre of gravity

Question 41

What is a Type 1A supernova? Explain how it happens:

Answer 41

-a type of supernova that occurs in binary systems in which one of the stars is a white dwarf, and the other can be any star -the white dwarf pulls material off the other star until it reaches 1.4M⊙ (Chandrasekhar Limit), and going above that means the electron degeneracy pressure in the star can't support the mass, so it collapses and goes supernova ## Footnote Due to this upper limit of mass, the luminosity of the explosion is fairly consistent

Question 42

How could you measure the distance to a star, without using parallax/redshift?

Answer 42

*Type 1A supernovae as Standard Candles* -measure the maximum apparent magnitude of a SN -as all SN's have the same constant luminosity, its absolute magnitude can be found from the HR diagram -calculate distance using distance modulus formula

Question 43

What is a pulsar?

Answer 43

-rapidly spinning neutron star that emits lots of **radio** waves from its magnetic poles due to its strong magnetic field -creates a "lighthouse" style rotating beam