beginners astronomy

BEGINNERS ASTRONOMY

Martin Crow Crayford Manor House Astronomical Society

THE GALAXIES AND COSMOLOGY


Last time

The types of stars and their properties.

Variable stars

Measuring the distances to the stars

The formation of stars


This week

The Milky Way – what is it?

Globular clusters.

Galaxies.

Galaxy clusters.

Cosmology and the ‘big bang’.


The Milky Way

The Greek philosopher Democritus (450–370 BC) proposed that the bright band on the night sky, known as the Milky Way, might consist of distant stars.

The Arabian astronomer, Alhazen (965–1037), made the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it was very remote from the Earth and did not belong to the atmosphere.

Others, including Aristotle, believed that it was "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere”.

Actual proof of the Milky Way consisting of many stars came in 1610 when GalileoGalilei viewed it through his telescope.

In 1750 Thomas Wright speculated (correctly) that the galaxy might be a rotating body of a huge number of stars. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. He also speculated that some of the nebulae visible in the night sky might be separate Milky Ways.


In 1755 Immanuel Kant introduced the term "island universe" for these distant nebulae.

The first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky.

In 1906 Jacobus Kapteyn used a similar, but more refined, approach of star counting to produce a size of 50,000 l.y. with the Earth still close to the centre.


A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kilo parsecs and the Sun far from the center.

M13 Martin Crow

Shapely used observations of RR Lyraevariables in the clusters (which he thought were Cepheids) to determine their distances.Because of this, and not taking into account theabsorption due to dust, his estimate for the size of the Milky Way was too large.

However, his discovery that the Globular clusters form a halo around the galactic core was correct.

This showed that the Sun was a long way away from the centre.

The debate over whether the Milky Way was the entire universe or that some of thenebulae were ‘island universes’ still continued.


In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to see if they showed emission spectra (expected if gaseous) or a continuum if stellar in nature.

He found that they were stellar and that some of them were red shifted giving speeds that indicated they might not be gravitationally bound to the Milky Way.

Martin Crow

In 1917, Heber Curtis had observed nova within the "Great Andromeda Nebula“ andfound that they were 10 times fainter than those that had been seen in the Milky Way.This suggested distances of around 150,000 parsecs.The matter was conclusively settled by Edwin Hubble in the early 1920s using a new Telescope (100 inch Hooker Telescope). He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.

So our Milky Way is one galaxy amongst many others.

The universe suddenly got a lot bigger.



The structural shape of our galaxy has been largely deduced from radioObservations in the 21 cm band.


So what we know about the Milky Way.

The Milky Way is our home galaxy

It is a barred spiral galaxy.

Its diameter is 100,000 l.y. and 1000 l.y. thick.

It consists of between 200 – 400 x 10⁹ stars.

The Sun is 26,400 l.y. from the centre.

The Sun takes 250 x 10⁶ years to complete one orbit of the galaxy.


Recent observations suggest that our galaxy has only two major arms making it looksimilar to NGC 1365.


We see it as an arc of cloud-like light across a clear dark sky.

Image by Julian Tworek


Image by Julian Tworek


Globular Clusters

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite.

Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.

Globular clusters are found in the halo of a galaxy

They are composed of old stars and there is no star birth going on in them.

The origin of globular cluster is still unclear.

There are 150 known globulars in the Milky Way halo.


Age estimates based on the coolest white dwarfs gives an age that may be as old as 12.7 billion years.Age and distance measurements can also be gained by plotting the stars ofa globular cluster on a Hertzsprung - Russell diagram.

Brightness comparisons of the main sequence stars give an idea of distance.The location of the ‘knee’ on the main sequence also gives an indication of age by looking at what stars still have to enter the red giant phase.


Galaxies

A galaxy is a massive, gravitationally bound, rotating system that consists of stars, stellar remnants, an interstellar medium of gas and dust.

Typical galaxies range from dwarfs with as few as 107 stars, up to giants with1014 stars, all orbiting the galaxy's center of mass.

All galaxies, it seems, have massive black holes at their centres.

The mass of the black hole at the centre of our galaxy is 4.1 x 10⁶ solar masses.

Black holes can be ‘quiet’ or ‘feeding’.

When feeding they emit copious amounts of x-rays as matter falls into them.

Galaxies with an active black hole are called AGNs (Active Galactic Nucleus).

Jets of relativistic material can also sometimes be detected streaming into the Interstellar medium.


Galaxies come in a variety of types: ellipticals, spirals, and irregulars.

A description of galaxy types based on their appearance is given by the Hubble sequence.


Ellipticals

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated.


Spirals

Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars.

In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. Our galaxy is an SBc.


Lenticular

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0 or SBo, and they possess ill-defined spiral arms with an elliptical halo of stars.


Interacting Galaxies

Interactions between galaxies are relatively frequent, and play an important role in their evolution.


Dwarf galaxies

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies


Galaxies come in groups

Notable galaxy clusters in the relatively nearby universe include the Virgo cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster.


There are a lot of Galaxies


Galaxy clusters can cause gravitational lensing.

Gravitational lensing is one of the predictions of Albert Einstein's General Theory of Relativity.

The mass of an foreground galaxy cluster can bend the light of distant objectscausing magnification and distortion of the background source.

SDSS


On the large scale the universe has a frothy look.


Measurements of galaxy rotation showed something odd.

The speed of rotation does not reduce with distance from the centre as predictedInstead it stays more or less constant.

Extra unseen mass would account for this ---- Dark Matter.

The mass of dark matter can also be calculated from its observed effects on gravitational lensing.


It is currently thought that all of the Baryonic matter (protons, neutrons, electrons)only accounts for 4% of the mass of the universe.

The rest of the missing mass is 74% dark energy.

Currently no one has a clue what it is.

Dark matter would appear to account for another 22% of the mass of the universe.


The Hubble constant

Following on from Slipher’s discovery of galaxy red-shifts Edwin Hubble discoveredthat a relationship existed between the red-shift of a galaxy and it’s distance.

The higher the red-shift the more distant galaxy.

The Hubble constant H₀ = 72 (km/s) / Mpc (depending on which data is used).


Edwin Hubble’s discovery of the receding galaxies implied that the universe wasexpanding.

An expanding universe was a property of Einstein’s theory of general relativity whichhe did not believe, as the universe at that time was thought to be static.

If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This lead to the idea that the universe started from a hot , dense state – The ‘Big Bang’.

Einstein introduced a term so that his equations kept the universe static. Later he referred to this as his greatest blunder.

The term big bang was coined as a ‘put down’ by Fred Hoyle who was opposed tothe idea.


The big bang is currently the favoured theory for where the universe came from.

According to this theory everything started from a singularity. This was the universe!!!


The big bang accounts for:

The abundance of elements in the early universe:

Hydrogen – mostlyHelium - some

Lithium – very small amount

The microwave background.


It has stood up quite well to new and more refined observations.

beginners astronomy

Documents

milky ways parallax

distant stars

long way

separate milky ways

resulting disk of stars

galaxy clusters

huge number of stars

night sky