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The Milky Way: General Structure

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1 The Milky Way: General Structure
Swinburne Online Education Exploring Galaxies and the Cosmos The Milky Way: General Structure Wait … loading image Activity: Observing the Milky Way The Milky Way star clouds of Sagittarius in the direction of our Galaxy’s centre © Swinburne University of Technology

2 Summary 1. To assess our Milky Way Galaxy from our location within it.
2. To use the knowledge (of only this century) that our Galaxy’s properties may be anticipated from, and compared with, properties of galaxies outside our own. 3. A final visual appreciation of our Galaxy before we rely on Observatory images. Title screen image: The wide, bright, central (Sagittarius) region of our Galaxy. Although finer detail photos are available, the author included his own photo here as an example of a standard SLR camera image. 60 sec guided exposure on 1600 film.

3 Introduction Galaxy studies take us from naked eye familiarity to images seen only by the world’s largest telescopes. This is our last chance (as we move out into the wider Universe) to consider some naked eye views of the night sky.

4 Naked eye views of the Milky Way
To any night sky observer, especially away from city lights, the normal random distribution of stars is broken by the band of light (and increased star density) we call the Milky Way. Casual observations of the Milky Way at different times and dates give confusingly different orientations of its band of light relative to the horizon, yet it has a simple overall general ‘shape’. Let’s look at some views of the Milky Way to discover its full extent.

5 Milky Way near Southern Cross
Star count falls off with distance away from the Milky Way This is a tree In this image, a torrent of stars cascades from the Southern Cross down to the horizon - the Milky Way. Note the dark ‘Coal Sack’ area below a Crucis. a Centauri, the closest star Note other dark features Southeast, 10pm, early March, from southern Australia The tree and the Coal Sack have something in common - they are both foreground features Follow this link for background material on estimating the distance to a Centauri

6 The Milky Way near Scorpius
Here we use computer software to show Scorpius and Sagittarius rising above the eastern horizon in May from southern Australia. Note the grey contours (isophotes) indicating the brightness of the Milky Way. 50o field of view This region is the brightest, widest mix of stars and glow of the Milky Way.

7 The Milky Way near Orion
The western horizon, with Sirius, through Orion to the Hyades and Pleiades clusters, setting in April from Southern USA. Again, note the grey isophotes of the Milky Way. 80o field of view

8 All Sky Milky Way views All sky (fish eye) view showing the Milky Way arching across the sky (centred by the bright Sagittarius region). At a different time the fainter half of the Milky Way (centred by the Orion region) completes an evident ‘ring’.

9 Milky Way around the Horizon
Sagittarius, Scorpius region Antares Milky Way around the Horizon LMC SMC At the latitude of Sydney, Australia, just before midnight in early October, the Milky Way lies entirely around the horizon - a great circle on the celestial sphere. Orion region

10 Great Circle The Milky Way thus has the simple overall general shape of a great circle ring of faint light and increased star density, with dark features and an increased brightness and bulk in the direction of Sagittarius. We observe from our particular location, on a tilted Earth, orbiting the Sun in a different plane to that of the Milky Way. No wonder its different orientations in the sky can be initially confusing. Activity: Observe the Milky Way’s orientation and extent over a few hours.

11 Alternative Structures
The ring of bright and faint stars we see as the Milky Way could either be a real ring or a disk of stars, with the Sun at or near the centre. Measurement of a wide range of distances to stars established that we are looking out through a disk of stars, like ants looking out at salt grains scattered over a transparent plate. Follow this link to background material on Magnitudes and Distances

12 Historical: The extent of the ‘Universe’.
At the start of this century, all extended regions of misty light were called nebulae (latin - ‘clouds’). Some of these evidently were associated with star clusters whose distance could be estimated. Were all of these nebulae within our Milky Way ‘Universe’? - including those with ‘spiral’ features and detail unresolved by telescopes of the day?

13 The Magellanic Clouds LMC SMC 47Tuc globular cluster Author’s photo to show close-to-naked-eye view in relation to horizon. 35o field, 50mm camera lens, 400ASA, 5 minute guided exposure. The naked eye Large and Small Magellanic Clouds never set for observers south of latitude 25o S. They are 33o and 45o (respectively) from the plane of the Milky Way. (See previous Map). They must have hinted at the fact - now known - that they are star systems, external to, but gravita-tionally bound to, the Milky Way. The properties of Cepheid variables in the LMC and SMC lead to their use as distance indicators for work by Hubble and Shapley. Follow this link to background material on using Cepheid variables

14 The Milky Way - One galaxy amongst many.
With the 100” telescope in 1924, Edwin Hubble identified Cepheid variable stars in the ‘Great Nebula in Andromeda’ (M31*) showing it to be far outside our own star system. The term ‘galaxy’ now means an isolated assembly of millions (to billions) of stars, gas and dust. The Milky Way comprises our own Galaxy (deserving capital ‘G”). Which type of galaxy is our Milky Way, from the evidence from our view from within it? ? ? ?

15 Viewing a spiral galaxy face-on.
Can we match the bright centre and spiral features of a galaxy like this to what we see in our own Galaxy? AAT 008 M83 is some 20 million light years away and appears at magnitude 7.6 in Hydra. Its apparent size in the sky is about one-third that of the Moon. Note more distant galaxy Does our Galaxy have a bright central region (and can we see it)? Foreground stars in our own Galaxy

16 Historical: Was the Sun near the Centre of the Milky Way system?
In 1917 Harlow Shapley investigated the distribution of globular clusters* - which appear in an apparent spherical halo above and below the disk of the Milky Way. The Sun was not near the centre of the distribution. A third of the Milky Way’s globulars (some 150 are now known) were in fact in the direction of Sagittarius. Sagittarius Expected to share the same centre of mass as the globulars, the galactic disk was therefore not centred by the Sun. *Click here to find out about globular clusters

17 A side-on view of a galaxy.
AAT 023 NGC253 is a 7th magnitude galaxy in Sculptor. It is like M83 but suggestively side-on. Imagine our Sun is one of those billions of stars, at the location shown. Sweeping around our new celestial sphere, from the location shown, we would expect to see a great circle of higher star concen-tration; brighter toward the galactic centre or a nearby spiral arm - for us, the Sagittarius region. Away from the plane of the galaxy, we would expect fewer stars - as, indeed, is the case.

18 Galactic Gridlines Introduction
In the next frame we are going to look at another region of the Milky Way - around Aquila. We will also use that frame to add two great circles with which you are already familiar: - the celestial equator dividing the celestial sphere, - the ecliptic near which, solar system objects are found. Corresponding approximately to the circle of the Milky Way, another great circle - the Galactic Equator - will also be shown. Try to get a feel for the angles between these three important planes.

19 Milky Way near Aquila Ecliptic Galactic Equator Celestial Equator All great circles (including the horizon) are shown with 30o tick intervals From Earth’s Equator, looking east at 10pm mid June, the Milky Way at Aquila is divided by a dark rift.

20 Historical: Was the Milky Way really a disk of stars?
Late last century, a disk theory was nearly abandoned before the improvement in telescopes and photography. It was thought that the dark rifts (as in the Aquila frame) in the Milky Way and dark patches (such as the Coal Sack) indicated an absence of stars and that, if within a disk arrangement, it would be too coincidental that they lined up, like tunnels, with our (chance) line of sight. The discovery of the presence of gas and dust causing dark nebulae and equatorial rifts, and the visibility of similar dark lanes in other side-on galaxies (as in NGC253) revived the disk concept; not a uniform disk, but one with a central bulge and spiral arms.

21 Galactic Equator 30 330 60 90 21h 19h 17h 300 270 X Note: All coordinate values increase eastwards - toward the eastern horizon East 120 60 SE NE Working again with the Aquila region, we now add coordinates to the great circles shown ... Click for: Horizon Celestial Equator (2hrs = 30o) Ecliptic Galactic Equator X = centre

22 The galactic coordinate grid
Galactic longitude (l) is measured in degrees eastwards around the galactic equator from 0o in Sagittarius. N (Coma Berenices) Galactic latitude (b) is measured in degrees North (+ve) and South (-ve) of the galactic equator. b Well away from the plane of the Milky Way we have views, clear of stars (and gas and dust), to distant galaxies and clusters of galaxies - for example in Coma Berenices and Virgo (near the North Galactic Pole) and Sculptor and Fornax (near the South Galactic Pole). l Sagittarius S (Sculptor)

23 Galactic Centre Features
M16 Eagle Nebula Hubble Space Telescope Image From AAT047 image X Galactic longitude 0o, a ‘spout’s’ length off the ‘teapot’ Galactic Centre Features M20 Trifid M8 Lagoon M17 Swan Tail of Scorpius Here are some of the notable features in the direction of the Galactic Centre Sagittarius (teapot) M6 M7 (naked eye cluster) Note the dark material (but not dark matter!) hiding our view through to the Galactic centre Use arrow keys to step back and forth to revise

24 Summary There are three broad regions of our Galaxy:
A disk, thin compared with its lateral extent, A brighter, wider central region or bulge, A halo of globular clusters (and, as we will see, stars and possibly dark matter.) The next Activity will look in detail at the type and distribution of stars in these regions - and the spiral structure must wait for later Activities. The next frame shows the dimensions of the Galaxy that our next Activity will reinforce.

25 The dimensions of our Galaxy.
The Sun is located about 8000 parsecs* (8 Kpc) from the centre. Here we examine a side-on view: . Spherical halo Globular clusters, stars, dark matter (?) Thin disk 2 Kpc 8 Kpc 0.6 Kpc Central bulge 50 Kpc The disk and bulge contain at least 100 billion stars Exercise: Review these dimensions in light years, given: 1 pc = 3.26 ly *(Remember: 1 parsec = million million km)

26 Image Credits AAT © David Malin (used with permission): M AAT028 NGC253 AAT003 Individual AAT images, © David Malin (used with permission), shown with a 6 character code - such as AAT028 - are found at the website ending with that code; eg: © the Author: Southern Cross region Sagittarius region Magellanic Clouds Use of output from the program GUIDE 7.0, courtesy of its author:

27 Hit the Esc key (escape) to return to the Index Page
In the next Activity, we will look at the range in luminosity of stars in our region; the existence of two broad Populations of stars and how this, and the existence of interstellar extinction, confused early estimates of the size of our Galaxy and the distance to other galaxies. Hit the Esc key (escape) to return to the Index Page


29 *Click here to find out about arcsec (”)
background Revisiting the parsec What is the significance of the distance unit called the parsec? Viewing the Earth’s orbit face-on from a star one parsec away, the Earth-Sun separation* would be 1 arcsecond* (1”). *Click here to find out about arcsec (”) Note: Diagrams such as this give a false impression of ‘nearness’ of stars. The length of the triangle should be 206,265 times the width! * 1 astronomical unit (AU) = 149,597,870 km. 1 parsec = million million km!

30 Distance determination (i) - by trigonometry.
background Distance determination (i) - by trigonometry. No star is as close as one parsec (pc). a Centauri is 1.3 pc. a d We can measure the same (parallax) angle a which the star appears to move against background stars, for two dates, 3 months apart, in our orbit around the Sun. A star at double the distance would halve the parallax angle. In general d (parsecs) =1 / a (arcseconds) Note: The tiny angle (a=0.76”) between Sun and Earth from even the closest star (a Cen) emphasises the difficulty in detecting planets which may be over 20 magnitudes fainter than their parent star.

31 The small angle formula.
background The small angle formula. A frequently used formula in astronomy applications. The distance around a full circle of radius r for a sweep angle of 360o is 2pr. r a s The arc length s is the same part of the full circle 2pr as the angle a is to ie: s/2pr = a/360 or: s = ra p/180 when a is in degrees or: s = ra when a is in radians or: s = ra / when a is in arcseconds For small angles, this also gives the straight line (chord) length.

32 Parsecs and Light years.
background Parsecs and Light years. The relation between the two distance units. By definition of a parsec, in the formula s = ra” / , a=1” and s=1 Astronomical Unit (149,597,870 km). So one parsec (r) = million million km. One light year is the distance light travels in a year at 300,000 km per second. Multiply by 60x60x24x365 for a year, giving one light year = 9.46 million million km. So 1 parsec = light years and 1 light year = parsecs.

33 Small angle formula applications.
background Small angle formula applications. Using: s = ra / with a in arcseconds 1. What is the actual distance (r) to a Centauri? Parallax a=0.76”, Earth-Sun baseline s = 150 million km, so distance r = 1.5x108 x / = ~41 million million km. s a r 2. What is the angular size (a) of a Centauri, assuming it is a star of similar size to the Sun? (s = 1.4x106km) a=1.4x106 x / 4.1x1013 = 0.007” ! Note: That’s why we don’t see ‘size’ in stars. Click here to return to the Activity

34 Click here to return to the Activity

35 Apparent magnitude and brightness.
background Apparent magnitude and brightness. By its modern definition a difference of five magnitudes implies a brightness factor of exactly 100. ie: for m2 - m1 = 5, b1/b2 = 100 or 102 2 1 In general, b1/b2 = 10 2/5 (m - m ) or (m - m ) 2 1 Then: log* b2/b1 = 0.4(m1-m2) or: m1-m2 = 2.5 log b2/b1 The globular cluster w Cen, of >100,000 stars, appears as magnitude 3.5. To a first approximation (similar stars, not masking others), what is their individual magnitude? Answer: m = 2.5 log 105 = 2.5x5 so m1 = 16 Note: The log key is provided on scientific calculators 10x *Click here to find out about logarithms

36 Brightness and distance.
background Brightness and distance. How would a star’s brightness (and magnitude) change if we, say, doubled its distance? A small surface area (such as a telescope mirror) collecting light from a star, is just a part of the surface area 4pr2 of the sphere of radius r. r With changing distance the light falling on the same collecting area will therefore change inversely with the square of the distance to the star. This is the inverse square law. So doubling the distance would quarter the brightness - a magnitude change of m1-m2=2.5log4 or 1.5

37 background Absolute Magnitude. The inverse square law allows us to recalculate a star’s brightness if the star was at a different distance. If we could consider all stars at some fixed distance, we could compare their real brightnesses. The fixed distance is chosen to be 10 parsecs. A star at distance d with apparent magnitude m, will, if moved to distance 10 parsecs, appear at what we call its absolute magnitude M. inverse square law m - M = 2.5 log b10/bd = 2.5 log (d/10)2 = 5 log d - 5 m - M is called the distance modulus.

38 Absolute Magnitude example.
background Absolute Magnitude example. Compare the real intensity of Sirius with that of Rigel. Using m - M = 5 log d - 5 The brightest star Sirius, with m=-1.4 is only 8.6 light years away. Rigel appears fainter at magnitude 0.2 but is 770 light years away. (Apply 1 parsec=3.26 light years) What are their true relative brightnesses? Sirius: M=-1.4-5log8.6/ =1.5 Rigel: M=0.2-5log770/3.26+5=-6.6 bRigel/bSirius =10 0.4(1.5-(-6.6)) = 1,738 times brighter!

39 Distance determination - ii).
background Distance determination - ii). Absolute magnitude can be inferred from a star’s intrinsic properties. Distances obtained from trigonometric parallaxes are useful to about 500 parsecs - and now improved by Hipparchus satellite measures. We can discover the brightness of a light bulb from the wattage written on it! A star gives away its absolute Magnitude by its colour or spectrum characteristics - its place in the H-R diagram. C O L O U R Absolute Magnitude Observing a star’s apparent magnitude and colour, and deriving M, leads to its distance, using m - M = 5 log d - 5 This is called the spectroscopic parallax method of distance determination and may be accurate only to within about 50%.

40 Distance determination - iii).
background Distance determination - iii). Stars in a cluster add weight to its distance determination. The non-evolved portion of cluster stars in the H-R diagram should fit the line of the main sequence. C O L O U R magnitude The apparent magnitude of the cluster is, of course, much fainter. M The correction to fit the main sequence is the distance modulus m - M giving 5log d - 5 and hence the cluster distance. m This distance measurement method is called main sequence fitting - a variation of spectroscopic parallax for individual stars. Click here to return to the Activity


42 Distance determination - iv).
background Distance determination - iv). Cepheid variable stars. The wonderful discovery, by Henrietta Leavitt in 1907, that the brighter variable stars in the Large Magellanic Cloud* (of the same type as d Cephei) had the longer periods, provided a method to measure distances to galaxies millions of light years away - provided these very luminous stars could be detected. A Cepheid variable’s observed period leads to its absolute Magnitude which, with its known apparent magnitude, leads to its distance using log d=(m-M+5)/5 Period in days Absolute magnitude *The LMC is over 160,000 light years away, so relative brightness differences in its stars are true differences. Henrietta Leavitt subsequently confirmed the relationship for Cepheids in the Small Magellanic Cloud. Click here to return to the Activity

43 Click here to return to the Activity


45 background Arcsec Arcsec (symbol “) is short for seconds of arc and is a measure of angle. A full circle is defined as an angle of 360 degrees. Each degree (o) includes 60 arc minutes (‘). Each arc minute (‘) includes 60 arc seconds (arcsec, “). Take 1/360th of a circle So one arcsec is one-sixtieth of one-sixtieth of one-three-hundred-and-sixtieth of a circle … a very small slice or the circle indeed. There are arcsec (“) in a full circle. Take 1/60th of that Take 1/60th of that No, you can’t see an arcsec in a diagram like this: it’s much too small a slice! Back to the Activity

46 background Back to the Activity


48 10 2 4 8 6 4 12 36 102 306 Arithmetic Geometric Logarithmic scales
background Logarithmic scales 10 2 4 8 6 A logarithmic view in mathematics uses a different way of making sequences of numbers. Instead of looking at what happens when you increase from 1 to 2 to 3, you consider instead what happens when you go from 1 to 10 to 100. In the 1, 2, 3 view, the increase is made by adding. The result is an arithmetic progression. Here’s another one. 4 12 36 102 In the 1, 10, 100 view, the increase is made by multiplying. The result is a geometric progression. The second way of increasing can help a lot when drawing graphs. Here’s a sample. 306 Geometric

49 background An example Why on Earth (or off Earth, in the case of your studies) would anyone want to use a logarithmic method? Let’s consider our tame astronomer who wants to illustrate the amount of money he owns (or hopes to own) at particular ages of his life.

50 background Not much good Trouble is, when he draws a graph of Money versus Age, the detail near the beginning isn’t clear. This is because the vertical scale has to go up to $100,000, so amounts like $1, $5 and $10 are too tiny to show.

51 Click here to return to the Activity
background The solution The solution is to draw the graph again, but this time he uses a logarithmic vertical scale where you increase by multiplying by 10 rather than by adding $10,000. A scale that goes up by multiplying rather than by adding is called a logarithmic scale. It is particularly useful when drawing graphs in astronomy, where figures can vary so very widely. Click here to return to the Activity

52 Click here to return to the Activity
background Click here to return to the Activity


54 Globular Clusters Globular Clusters are dense concentrations of stars
background Globular Clusters Globular Clusters are dense concentrations of stars - up to a million of them bound together by their mutual gravity Their total mass is so large that gravity pulls the cluster into the shape of a sphere … or globule … hence the name GLOBULAR clusters

55 What if we lived in a Globular Cluster?
background If the Sun were to have evolved within a Globular Cluster we would be surrounded by thousands of stars within the nearest parsec alone. That’s a far cry from the zero stars that share the cubic parsec surrounding our Sun today. The sky would be so bright with stars at night, assuming there was a night…. (most stars would have companions) …that we might never realise there was more to the universe than our small cluster of stars. Return to the Activity

56 background Return to the Activity


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