Topic D- Astrophysics

D.1- Stellar Quantities, D.2- Stellar Characteristics & Stellar Evolution, D.3- Cosmology

Light Year and Astronomical Unit

• Light year- distance that light travels in a year

• AU- estimate of the distance from Earth to the Sun and approximately equal to 150 million kilometres

@@D1.1 OBJECTS IN THE UNIVERSE@@

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@@D1.1@@ Comet

A small body (mainly ice and dust) orbiting the Sun in an elliptical orbit

D1.1 Stars

• Single Star: a ball of gas that undergoes nuclear fusion (don’t orbit anything, held together by its own gravity)

• Binary Star: Two stars orbiting a common centre

@@D1.1@@ Planetary Systems

* Solar System made up of 8 major planets
* All planets revolve around the sun in the same direction

@@D1.1@@ Constellations

A group of stars in a recognisable pattern that appear to be near each other in space

@@D1.1@@ Stellar Clusters

* **stellar clusters:** groupings of large numbers of stars that attract to each other gravitationally and are relatively close to one another
* **globular clusters:** large clusters of mainly old evolved stars
* **open clusters: a** smaller number of younger stars that are further apart
* **galaxies (in both clusters):** a large number of stars and stellar clusters
* **superclusters:** clusters of galaxies

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(each diagram/dot point is further out in space)

@@D1.1@@ Nebulae

Clouds of ‘dust’, i.e. compounds of carbon, oxygen, silicon and metals, as well as molecular hydrogen, in the space in between stars

D1.1 Galaxy

A collection of a very large number of stars mutually attracting one another through the gravitational force and staying together.

The number of stars in a galaxy varies from a few million in dwarf galaxies to hundreds of billions in large galaxies. It is estimated that 100 billion galaxies exist in the observable universe

@@D1.1@@ Cluster of Galaxies

Galaxies close to one another and affecting one another gravitationally, behaving as one unit

@@D1.1@@ Super Clusters of Galaxies

Collections of clusters of galaxies further out in the galaxy

@@D1.2 THE NATURE OF STARS@@

* the reason why stars don’t collapse is because there’s opposing pressures
* radiation and gravity

@@D1.3 ASTRONOMICAL DISTANCES@@

* unit of distance:
* *1 light year= 9.46 x 10^15m* (formula booklet)
* *Average distance between stars= 1 pc (parsec)*
* *4.2 ly=1.3 pc*
* *Distance of parsec= 1/p(parsec)*

@@D1.4 STELLAR PARALLAX@@

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@@D1.4@@ Parallax

means of measuring astronomical distances that takes advantage of the fact when an object is viewed from two different positions it appears displaced relative to a fixed background

@@D1.4@@ Parallax Formula

*d=R/p or d=R/tanp*

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(measured in radians)

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or *parsec= 1/p* (in arc seconds)

@@D1.5 LUMINOSITY AND APPARENT BRIGHTNESS@@

@@D1.5@@ Luminosity

* the power radiated by a star measured in watts
* stars are assumed to radiate similar to black bodies

D1.5 Luminosity Formula

L = σAT^4

AREA= surface area of a star therefore 4pie r squared

(in formula booklet)

@@D1.5@@ Finding temperature/brightness from luminosity formula

b= σAT^4/4πd^2

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(in formula booklet)

D2.1 STELLAR SPECTRA

* the energy radiated by a star in the form of electromagnetic radiation and is distributed over an infinite range of wavelengths

D2.1 Wien displacement law

λmax T = 2.90 × −10^3 mK

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the relationship wave length of maximum intensity radiation of a star and its temperature

D2.1 STELLA CLASS

stars are divided into 7 spectral classes according to colour:

* OBAFGKM

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(O is blue (hottest) ---→ M is Red (coldest))

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Acronym: oh be a fine girl kiss me

D2.2 THE HERTZSPRUNG-RUSSELL DIAGRAM

remember shape, white dwarfs and red supergiants

D2.3 MAIN-SEQUENCE STARS

A normal star that is undergoing nuclear fusion of hydrogen into helium. Our Sun is a typical mainsequence star

D2.4 RED GIANTS AND RED SUPERGIANTS

A main-sequence star evolves into a red giant – a very large, reddish star. There are nuclear reactions involving the fusion of helium into heavier elements

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supergiant= just bigger

D2.5 WHITE DWARFS

The end result of the explosion of a red giant. A small, dense star (about the size of the Earth), in which no nuclear reactions take place. It is very hot but its small size gives it a very low luminosity

D2.6 MASS-LUMINOSITY RELATION

For mainstar sequence only

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Formula: *L (fish) M^3.5*

D2.7 CEPHEID STARS

when stars do not have a constant luminosity in time but varies periodically from a minimum to a maximum

D2.7 Why a Cepheid Star Varies in Luminosity

The reason for a periodic variation is the periodic expansion and contraction of the outer layers of the star

• if asks about size- relates to surface area of outer layer

Process:

1. periodic relationship in size from outer layers

2. leads to periodic increase in temperature

3. then periodic increase in luminosity

D2.8 STELLAR EVOLUTION

* Initially stars have a low surface temperature and they start right of the main sequence on the HR Diagram

D2.8 Protostars

Form when sections of giant molecular clouds start to collapse

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(basically they come before normal stars)

D2.8 Main Sequence

As a star contracts it begins to move towards the main sequence

* heavier stars take less time

D2.8 Low Mass Stars

* Helium collects in the core of the star surrounded by hydrogen
* Only hydrogen in the inner shell undergoes nuclear fusion to helium

D2.8 Planetary Nebula

when there is a huge release of energy that blows away outer layers of the star in an explosion

D2.8 High Mass Stars

* evolutionary path of 15 solar masses
* less than 1.4 it becomes a white dwarf
* 2-3 it collapses and forms a neutron star

D3.1 HUBBLES LAW

States that the velocity of recession is directly proportional to the distance (d) between the Earth and the galaxy’s velocity (v) of recession.

v=Hod

Ho=hubble’s constant

doppler effect indicates galaxies are moving away from us. Which proves there are red-shifted absorption lines

D3.2 Cosmic Scale Factor R + Red Shift

The expansion of the universe can be described with the scale factor, R. This is the difference in coordinates of ∆x by R gives the physical distance between to point

d=R∆x

D3.3 Big Bang Model

• The Big Bang theory states that both space and time originated with the expansion from a singularity.

• The evidence that supported the Big Bang theory was observed through the redshift (Doppler effect) of almost all the galaxies. This indicates that all of the galaxies are moving away from us.

D3.3 Evidence to Big Bang Model

1. The early universe was in thermal equilibrium and the radiation from then had a black body spectrum, which has traveled through space, becoming increasingly redshifted up to this point in time. This reduces the temperature of the black body spectrum and the radiation should be visible from every point in space.

2. As the radiation travels throughout the universe, space has expanded, causing the wavelength to increase and its energy to decrease.

D3.4 CMB (Cosmic Microwave Background)

• The discovery of the CMB (Cosmic Microwave Background) led to the Big Bang theory becoming the currently accepted model since it is not supported by the Steady State theory

  • The CMB is a type of electromagnetic radiation which is a remnant from the early stages of the Universe

  • It has a wavelength of around 1 mm making it a microwave, hence the name Cosmic Microwave Background

D3.5 Accelerating Universe and Red-Shift

The evidence for an accelerating expansion comes from observations of the brightness of distant supernovae. We observe the redshift of a supernova which tells us by what the factor the Universe has expanded since the supernova exploded. This factor is (1+z), where z is the redshift. However, in order to determine the expected brightness of the supernova, we need to know its distance now. If the expansion of the Universe is accelerating due to a cosmological constant, then the expansion was slower in the past, and thus the time required to expand by a given factor is longer, and the distance now is larger. But if the expansion is decelerating, it was faster in the past and the distance now is smaller. Thus for an accelerating expansion, the supernovae at high redshifts will appear to be fainter than they would for a decelerating expansion because their current distances are larger.