Life and Death of Stars - Part 1 Flashcards
What is our galaxy?
- an enormous collection of stars and interstellar matter
- ## contains over 100 billion stars spread through a volume of space close to 100,000 lightyears across
What is our galaxy held together by?
- GRAVITY
What do stars rotate around?
- the galactic centre
- around 250,000 away from Earth
How many stars are there thought to be in our universe?
- around over one quintillion (10 to the power 18) stars
What are some physical quantities of stars?
- luminosity (brightness)
- temperature (colour)
- chemical composition
- size
- mass
What is the photosphere?
- the ‘outer’ layer of the sun
- not such an outer layer, more an area where the internal energy of sun is released back into the atmosphere as visible radiation
- it is the sphere of light that we see on a sunny day
- surface temp = 5800 K
- thickness = 500 km
- has a churning granulated structure with a few dark spots called sun spots
- these also change in size and shape
- the churning, granulated structure is our view of the top of the solar convection
What is the corona?
- can be seen during a total solar eclipse
- extends out ti a distance of several times the radii of the Sun
- can be seen directly when X-ray telescopes are used
What is the chromosphere?
- between the corona and photosphere
- thickness = 1600 km
- more visually transparent than the photosphere
- during a solar eclipse, this is the pink / red ring around the moon
- temperature = 6000-10,000 K
Describe the interior of the Sun
- divided into 3 different regions
- based on how energy / heat is transferred at various points
- CONVECTION ZONE
- RADIATION ZONE
- SOLAR CORE
Interior of the Sun - Convection zone
- immediately below the photosphere
- causes the granular look of the photosphere
- heat is transferred here by convection
- hot material flows up to the surface
- cool material sinks down to the larger depths
Interior of the Sun - Radiation zone
- below the convection zone
- solar energy is transported from the core to the outer parts by radiation
Interior of the Sun - Solar Core
- the very heart of the Sun
- where the nuclear reactions which provide it with its energy occurs
- energy is also transported by radiation here but very slowly
What is luminosity?
- the amount of energy radiated by a star
Sun’s luminosity = 4 x 10 to the power 26 W
What is the solar constant?
- the amount of solar energy that reaches the surface of th Earth’s atmosphere every second
- is 1400 W m(-2)
- about 50-70% of this energy reaches the Earth’s surface
- the rest is absorbed by the atmosphere or reflected away by clouds
What is nuclear fusion?
- the way in which the Sun creates its energy
- the combination of light atomic nuclei into heavier ones
- two nuclei combine to create a third
- the mass of this third nuclei is always less than the total of the first two
- the lost mass has been converted into energy
- given by the equation E = mc(2)
What is nucleosynthesis?
- the building of elements through the process of nuclear fusion
Properties of the Sun
Radius = 7 x 10 to the power 8 m Mass = 2 x 10 to the power 30 kg Average density = 1410 kg m(-3) Surface gravity = 274 m s(-2) Time for rotation = 25 days (at equator) Surface temperature = 5780 K Luminosity = 3.9 x 10 to the power 26 W
Comparing the Sun with other stars - SIZE
- stars vary greatly in terms of size
- go from a few hundredth of a solar radius to several hundreds of a solar radius
Comparing the Sun with other stars - MASS
- there is no way to determine the mass of a single distant star
- the only way is to calculate stellar mass is to measure its gravitational effect on some other object
- this other object - often another star or a planet
- binary star system - two stars orbit each other
- its possible to work out their mutual gravitational attraction
- since this attraction is dependent on the masses of those stars, we can then work out the masses of those stars
Comparing the Sun with other stars - LUMINOSITY
- use luminosity and surface temperature to classify stars in the same way height & weight are used to classify humans
- therefore we can say that luminosity and surface temperature are connected
Hertzprung-Russell diagrams
- diagrams plotting the luminosities of stars against their surface temperatures
What is the main sequence?
- on H-R diagrams a band, containing a large concentration of stars, running from top-left to bottom-right
- Sun and the majority of it’s neighbours live on it
- temperatures = 3000 to 30,000 K
- luminosities = 10(-4) to 10(4) solar units
- radii of stars also varies
What is the Stefan-Boltzmann Law?
- the radiation emitted by a star is governed by this law which states that ‘the energy emitted per unit area per unit time increases as the fourth power of the star’s surface temperature’
- star’s luminosity is this energy multiplied by its surface area
luminosity is proportional to radius(2) x temperature(4)
What are blue giants?
- found at the end of the main sequence
- large, hot and blue stars
What are red dwarfs?
- found at the other side of the main sequence
- small, cool and faint stars
- the most abundant types of star found in the universe
- more than 80% of stars
What is the relationship between a star’s mass and luminosity?
- there is a clear relationship between the two
- low mass stars = bottom of the main sequence
- high mass stars = greater luminosity
the relationship between the luminosity of a star and its mass is roughly linear
What does the luminosity relationship allow us to work out?
- the relative lifespans of different stars
- once a star has burnt up all of its fuel, it dies
- to estimate how long this will last for, you divide the amount of fuel available by the rate at which the fuel is being used up
stellar lifetime is proportional to (stellar mass / stellar luminosity)
How can you tell which stars are hot and which are cool?
- by looking at their colour
- red stars = cool stars
- blue stars = hot stars
Properties of light - INTENSITY
- often used to specify the amount or strength of radiation at any point in space
- a basic property of radiation
- no natural object emits radiation at just one frequency
- energy is generally spread out over a range of frequencies
- how studying the how the intensity of this radiation is distributed across the electro-magnetic spectrum, we can learn lots about the object’s properties
What is a blackbody?
an object that absorbs all radiation falling on it
What is a blackbody curve?
- in a steady state, a blackbody must re-emit the same amount of energy as it absorbs
- the curve describes the distribution of this re-emitted energy
- peaks at a single, well-defined frequency and falls of to lesser values above and below that value
- the curve is not symmetrical!
- intensity falls off more slowly to lower frequencies than higher frequencies
- this overall shape holds for any blackbody object no matter the size, shape, composition or temperature
What are the two key features to the blackbody curve that help us to use it to understand the behaviour of real objects?
- as the temperature of the object increases, the frequency at which the distribution peaks also increases
- very hot objects glow visibly - they emit visible light! - as the temperature of the object increases, the amount of energy emitted increase
- cooler objects produce infrared (invisible) radiation
How can astronomers determine a star’s surface temperature?
- by measuring its apparent brightness at several frequencies
- then matching these observations to the appropriate blackbody curve
- since the basic shape of the blackbody curve is very well understood, it is possible to determine the temperature of distant stars with measurements at just two wavelengths
What two filters do we use to make measurements of a star’s surface temperature / brightness and relating it to blackbody curves?
- B (blue) filter - rejects all radiation apart from 380-480nm
- V (visual) filter - only passes light in the 490-590nm range
see lecture notes for diagrams!
What can we use a blackbody curve for?
- to estimate the surface temperature of a star
- the ratio of the B flux : V flus is often referred to as the colour index / colour of a star
Apart from colour, what is another useful way used by astronomers to gain information about stars?
SPECTROSCOPY
- provides information about the absorption lines of certain elements in a star
- the lines indicate which elements make up the star
Atomic structure of an H atom
- the simplest atom
- consists of a negative electron that orbits a proton which carries a positive charge
- the proton forms the nucleus of the atom
- an electrically neutral atom - the charges of the proton and electron cancel each other out
- the opposite charges produce an electrical attraction that binds them together within the atom
What happens when an atom absorbs some energy?
- the energy absorbed must cause some internal change to the atom itself
- if the atom emits energy, then that energy must come from somewhere within the atom
THEREFORE it is reasonable to suppose that the energy absorbed or emitted by the atom is associated with changes in the motion of the orbiting electron
The Atomic Bohr Model
- there is a state of lowest energy (the ground state) which represents the normal condition of the electron as it orbits the nucleus
- there is a maximum energy that the electron can have and still be a part of the atom
- once the electron acquires more energy than this, it’s no longer bound to the nucleus and the atom is ionised and becomes an ion
- between these two energies the electron can only exist in certain sharply defined energy states, known as orbitals
> each electron orbital is pictured as having a specific radius, like a planet’s orbit in a solar system
Modern picture of the H atom
- orbitals are not sharply defined
- the electron is envisioned as being smeared out in an electron cloud surrounding the nucleus
- atoms do not always remain in their ground state
- an atom is ‘excited’ when an electron occupies an orbital at a greater than normal distance from the nucleus - it has a greater than normal amount of energy!
- an atom can become excited in one of two ways:
1) absorbing some energy from a source of electromagnetic radiation
2) colliding with another particle
What are photons?
- packets of energy of specific values
- are absorbed by electrons when atoms absorb energy from a source of electromagnetic radiation
- the characteristic wavelengths of this absorbed radiation is determined by the internal structure of the atoms
- the internal structure of each element is unique to that atom so the absorption lines seen are characteristic of that specific element
- when an electron de-excites, it gives off energy in the form of photons
- the wavelengths of this emitted radiation is determined by the internal structure of that atom
Stellar spectra - Temperature
- some stars display strong lines in the long-wavelength part of the spectrum whilst others dis play strong lines in the short-wavelength part
- for others, the lines are spread throughout the spectrum
- these differences are due to the different temperatures of the stars!!!
Main differences in Stellar spectra - temperatures above 2500 K
- more than 25000 K - usually show strong absorption lines of singly ionised He and multiiply-ionised heavier elements (O, Ni, Si)
- not seen in cooler stars as only very hot stars have the energy to excite and ionise such tightly bound atoms
Main differences in Stellar spectra - H absorption lines
- in hot stars, these are relatively weak
- BECAUSE at such high temperatures, much of the H is ionised so there are few intact H atoms to produce strong lines
- strongest in stars having intermediate surface temperatures around 10,000 K - just right for electrons to move frequently between the orbitals producing the characteristic visible higher spectrums
- weak in stars with a surface temperature below 4000 K because there is not enough energy to excite the electrons from the ground state
Spectral Classification
- stars are classified according to their surface temperature
- in order of decreasing temperature; O, B, A, F, G, K, M
- these are then divided into 10 smaller groups
- denoted by the numbers 0-9
- lower number = the hotter the star
Luminosity class
- classifying stars according to the width of their spectral lines
- allows astronomers to distinguish between different classes of stars by studying a single spectral property (line broadening) of the radiation received
- by studying the widths of the spectral lines, we can get information on the star’s physical conditions
What are the different stellar luminosity classes?
Ia - Bright Supergiants Ib - Supergiants II - Bright giants III - Giants IV - Subgiants V - Main-sequence stars and dwarfs
What is luminosity?
absolute brightness!
- an intrinsic property of a star
- does not depend on the location or the motion of the observer
What is apparent brightness?
- what we are measuring when we look at a star
- the amount of energy striking a unit area per unit time
- a measure of the energy flux produced by the star (the amount of energy per unit area per unit time) as seen from Earth
- a star’s apparent brightness depends on its luminosity and our distance from the star!
What is the link between luminosity, apparent brightness and distance?
apparent brightness is proportional to (luminosity / (distance squared))
- if luminosity x2, the apparent brightness is 2x
- so two identical stars can only have the same apparent brightness if they lie at the same distance from Earth
- BUT two non-identical stars can have equal apparent brightness if the more luminous of the two is further away
What needs to be known if we want to determine a star’s luminosity?
- the star’s apparent brightness
(determined by measuring the amount of energy detected in a given time) - the star’s distance from us
Origins of the magnitude scale
- divides stars into 6 categories, numbered 1-6
- 1 = brightest stars, 6 = faintest stars
Modern magnitude scale
- a change of 5 in the magnitude of an object corresponds to exactly a factor of 100 in apparent brightness
- the numbers on the scale = apparent magnitudes
- the magnitude range has been extended beyond 1-6
What is apparent magnitude?
- measures a star’s apparent brightness when the star is seen at its actual distance from the Sun
- to compare intrinsic properties of stars, we pretend we are looking at all stars as if they were at a distance of 10 parsecs
What is absolute brightness?
- a star’s apparent magnitude when it is placed at a distance of 10 pc from the observer
- since the distance to the star is fixed in this definitions, absolute magnitude is a measure of a star’s absolute brightness of luminosity
Distances to stars and their motions through space
- problem - how do we apprehend how big the Universe is?
- to explain the Universe’s structure, we need to be able to measure distances to far away objects
- astronomers have come up with a range of methods / rules / techniques to measure the vast distance to fainter and fainter objects
What is parallax?
- the apparent shift of a foreground object relative to some distant background as the observer’s point of view changes
- to determine an object’s parallax, we observe it from either ned of some baseline and measure the angle through which the line of sight to the object shifts
- as the distance to the object increases, the parallax becomes smaller and therefore harder to measure
- LIMITED TO NEARBY STARS!
What are our neighbours?
the closest star to Earth is Proxima Centauri
then Barnard’s star
What is main-sequence fitting?
- a technique that allows us to measure distances to relatively far away stars
- fundamental to the technique = a measurement of apparent brightness of a light source combined with some knowledge of its intrinsic properties
- MS - represents a fairly close correlation between temperature and luminosity for most stars
- allows us to make a connection between an easily measured quantity - spectral types - and the star’s luminosity which would otherwise be unknown
MS-F - refers to the specific process of using stellar spectra to infer luminosities and hence distances
Summarise main-sequence fitting
- measure the star’s apparent brightness and spectral type without knowing how far away it is
- use the spectral type to estimate the star’s luminosity
- apply the inverse-square law to determine the distance to the star
Stellar motion
- as well as the apparent motion due to parallax stars, stars also have real spatial motion through the galaxy
stellar motion has two components:
- Radial velocity
- Transverse velocity
What is radial velocity?
- along the line of sight
- can be measured using the Doppler effect
What is transverse velocity?
- perpendicular to our line of sight
- can be used for many nearby stars by careful monitoring of the star’s position in the sky
What is proper motion?
- describes the transverse component of a star’s velocity relative to the Sun
- the annual movement of stars across the sky, as seen from Earth and corrected for parallax
- measured in terms of angular displacement
Binary star systems
- systems of two or more stars
- the stars are sufficiently close enough to exert mutual gravitational forces on each other
- this means they exhibit specific trajectories, as their motions will be determined by the gravitational forces they are experiencing
- a system where two stars not only appear close together in the sky, BUT actually are physically close from each other
- their physical separation is typically a few astronomical units rather than several parsecs
When can we detect a binary star system?
- can only know there is a binary when we detect evidence of the orbital motion of one star about the other
- can sometimes be detected when there is no immediate optical evidence of two stars (when they cannot be distinguished from each other by a telescope)
What is a visual binary?
- the simplest type of binary system and the most easily recognised as such
- have to study a pair of stars intensively before you can determine that it is a true binary
- a common proper motion of the two stars is often the first clue that the system forms a genuine binary system
- the two stars will be no more than a few arc seconds apart
How do we determine the motion of stars within visual binaries?
- need positional measurements of the highest accuracy
- simplest way = measure the relative positions of the two stars in the binary system
- the angular separation and the position angle of the stars is measured
What is an astrometric binary?
- binary systems that appear like single stars because one of the components is too faint to be seen
What are spectroscopic binaries?
- appears like a single star too
- no prospect of the two stars being seen separately
- it’s their close proximity to each other that causes them to appear as a single point of light
- their binary nature is revealed only through spectral analysis
What is an eclipsing binary?
- also is a spectroscopic binary BUT has a bonus addition of varying velocity curves as brightness varies periodically due to the eclipses taking place
- this variation in magnitude of an apparently single star can be graphed to produce a light curve, which will usually have a rather artificial appearance
- by closely examining the light curve of an eclipsing binary, we can infer:
> the individual brightness of the two stars
> what fraction of the brighter star is observed
> which in turn tells us the ratio of the areas of the two stars and hence the ratio of their radii
Inferring the mass of main-sequence stars
- the mass of a star is most easily obtained if it is a member of a binary system
- binary star systems are very common!
- for visual binaries and eclipsing binaries, the masses of both stars can be clearly established
- a complete mass determination is not possible for spectroscopic or astrometric binaries
What is an element defined by?
- the number of protons in its nucleus
What do all elements exist in?
isotopes
What is an isotope?
- two isotopes of any given element have the same number of protons in their nucleus but a different number of neutrons!
most common isotope = the ‘normal’ form of the element
How many stable and unstable chemical elements are there on Earth?
- 84 stable elements
- these make up the vast majority of matter in the universe
- 14 radioactive elements that occur naturally on Earth
What is radioactive decay?
- if a given sample of a radioactive element has N nuclei within it, then over time this number will reduce as the nuclei decay to a more stable entity
- decay constant = unique to the individual element
What is the half-life of an element?
- the amount of time it takes for half of the radioactive element’s nuclei to decay
- measuring the amounts of elements in rock samples and knowing half-lives allows the ages of samples to be estimated
How much radioactive stuff is there?
- the slow and steady decay of radioactive material over the 4.5 billion years since the formation of the solar system means they are scarce on Earth, in meteorites and in lunar samples
- not seen in stars - not enough of them!
- there are several other radioactive element that have been created artificially in laboratories or during nuclear weapons testing
What are the simplest elements in our universe?
H and He
- created very early in the universe’s history
- known as primordial
What is stellar nucleosynthesis?
- how other elements were created
- formed by nuclear fusion in the hearts of stars
