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110
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Astronomy
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Jan 9, 2024
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Computer Lab – Galaxies & Nebulae
(Virtual Lab Remote Edition)
Picture Gallery
The Voyager 4 program has an extensive archive of astronomical pictures. Unfortunately, when accessed through the CSUB Virtual Computer Lab, those pictures don’t display properly. So, instead I will fill this document with pictures at the expense of making this a rather large document size. Feel free to delete pictures to make the file size more manageable when you create your answer file for submission (some of the questions will refer to the pictures so don’t dump them too quickly).
The first half of this lab exercise will just use the included pictures. Only the second half will use the Voyager 4 program.
The Lives of Stars: Nebulae and Star Clusters
1.
A nebula is a gas cloud, nebulae are usually hydrogen and helium gases with small amounts of all other elements (same composition as our Sun and most stars). When the nebula is cold, the gases and dust become more dense and they scatter or absorb all the incident visible light; this is called a dark nebula
. That’s not a hole in space in the above image, it is a dark nebula which is blocking the view of the stars behind it.
Galaxies & Nebulae – 1
2.
Maybe the most famous dark nebula is the Horsehead Nebula seen in the center of the picture above. This is part of the famous Orion Nebula, we will explain the pink fog and blue-white object in the lower-left in later pictures.
Galaxies & Nebulae – 2
3.
Above is a picture of the Eagle Nebula. We see more of that pink glow, let’s explain that now. Excited hydrogen atoms emit the four wavelengths of visible light, shown above. When humans receive those four colors together, they register as pink. Since hydrogen is the most common element in the universe, you will see that pink color again and again in astronomical pictures. The pink glowing clouds of hydrogen gas are called “emission nebulae”.
Galaxies & Nebulae – 3
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4.
This is a Hubble Space Telescope picture of the center of the Eagle Nebula, these dark nebulae have been dubbed the “Pillars of Creation”. Gravity causes dark nebulae to collapse and form into stars. A large dark nebula will fragment and form into a cluster of stars. Each little finger or spike sticking out from the Pillars will become a star.
5.
This is a picture of the Pleiades (or Seven Sisters), a young cluster of stars. The Orion Nebula and Eagle Nebula will likely evolve into clusters resembling the Pleiades. While the Pleiades contains at least a 1000 stars, the light we receive is dominated by relatively few hot blue supergiant stars. When their blue-white light reflects off of gas clouds, we call that a “reflection nebula”.
Galaxies & Nebulae – 4
6.
The Trifid Nebula is a combination of star cluster, emission nebula (pink), and reflection nebula (blue).
7.
The Jewel Box cluster is another very young cluster with most of its light coming from a few super-luminous supergiant stars.
Galaxies & Nebulae – 5
8.
The Hyades star cluster is an older cluster of stars, estimated to be about 625 million years old. Likely the cluster used to have supergiant stars but those stars live for only a few million years and are long gone by the time the cluster gets as old as the Hyades. The brightest stars in this cluster are the red giants and yellow giants (stars that used to be high-mass main-sequence stars).
What about the clusters that are as old as our Sun, 4.5 billion years old? Well, over time the clusters we’ve been talking about (“Open Clusters”) lose members and spread apart until there is no longer any obvious cluster. Astronomers are still trying to pin down where the Sun was born and who were our sibling stars.
9.
This is the star cluster called M13, it is a “globular cluster” as opposed to an open cluster. It contains over 100,000 stars in a sphere of diameter just 170 light-years. The cluster and the stars in it are estimated to be 11.65 billion years old. The picture to the right is a close-up of the center, almost all the stars are red, orange, yellow, or yellow-white; they are mostly low-mass main-sequence stars. A few blue stars have been found in globular clusters (called “blue stragglers”), they are believed to be new stars created by the collision and merger of other stars.
Galaxies & Nebulae – 6
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10.
The Milky Way contains at least 150 globular clusters, they are believed to have formed back when the Milky Way formed. The picture above is the Tarantula Nebula, an enormous (almost 2000 light-years across) star-forming nebula in the Large Magellanic Cloud (a small galaxy that orbits our Milky Way). The Tarantula is expected to eventually coalesce into a rare, newly-born globular cluster.
11.
High-mass stars live short, violent lives. This is Eta Carinae, at the center is a pair of stars, one of about 200 solar masses and the other about 50. Past eruptions from the stars have created the Galaxies & Nebulae – 7
shells and sheets of gases seen in the picture spreading out into space. In 1837 the star suddenly became 100 times brighter to become one of the brightest in the night sky. From March 11-14, 1843, it was the second brightest. From 1856 to 1940, it was not visible to the naked eye. Eta Carinae is expected to explode in a supernova but no one knows exactly when that will occur.
12.
Sun-like stars will not explode in a supernova. They will progress from main-sequence to red giant to yellow giant to planetary nebula to white dwarf. The above montage is pictures of planetary nebulae. In their dying gasps, stars can spew most of their mass out into space. It appears to be common for stars to expel gas out into an hourglass-like shape. Why? Maybe these stars have orbiting companion stars that restrict the outflow of gas. When seen from an angle, that can look like side-by-
side loops or when seen end-on as a single ring.
In most of the pictures you can see a small white dot at or near the center. That is the white dwarf remnant of the star, all that will be left after the planetary nebula gases disperse into space.
Galaxies & Nebulae – 8
13.
This is the Crab Nebula, a supernova remnant. After they finish their short supergiant lives, the most massive of stars die in a supernova explosion. This shows most of the mass of the original star flying outwards. At the center (too small to be seen) is a neutron star, the “Crab Pulsar”. Larger stars can leave behind a black hole. White dwarfs, in some rare circumstances, can also explode in a supernova (those explosions don’t leave any remnant).
14.
This is the supernova remnant 1987A, the nearest supernova since the invention of the telescope. The rings are the result of the gases jetting out from the supernova colliding with gases ejected from earlier eruptions.
Galaxies & Nebulae – 9
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Galaxies
15.
Galaxies are typically divided into three main categories: spirals, ellipticals, and irregulars. The spiral galaxies come in two sub-classes, regular and barred spirals. Our Milky Way is a barred spiral, looking much like the second galaxy in the second row. Both the spiral arms and the bars across the center are believed to be the result of “density waves” but we won’t attempt to understand those details.
Galaxies & Nebulae – 10
16.
Our Milky Way is part of a cluster of galaxies known as the “Local Group”. This cluster is about 10 million light-years across and contains at least 80 member galaxies (mostly “dwarf” galaxies, either irregular or elliptical). The Andromeda Galaxy is the largest member of the Local Group and the Milky Way is the second largest; most of the dwarfs are clustered around on or the other of those two. Our Local Group is part of the larger Virgo Supercluster of galaxies.
Galaxies & Nebulae – 11
17.
Strange radio wave sources were discovered starting in the late 1950s. Optically they looked star-like, they were named quasi-stellar radio sources, or quasars for short. Evidence showed that they were incredibly distant which meant they were emitting astounding amounts of radiation, mysterious.
When stars wander too close to a black hole, it tears them apart, energizing all their gases, before swallowing them. The answer to the quasar mystery was black holes, but not ordinary black holes. All galaxies, even our Milky Way, are believed to have supermassive (millions or even billions of times more massive than our Sun) black holes at their centers.
The power source of quasars are supermassive black holes in a frenzy feeding on hapless stars. Quasars are a common phase early in the life of galaxies (our Milky Way was likely one once) but become rare later on when there is less left for the monster black hole to eat. I believe the above illustration is showing a quasar way back in the early days of the galaxy when mass of the galaxy was mostly dark nebulae.
18.
Galaxies & Nebulae – 12
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This picture is a small portion of the “Hubble Ultra-Deep Field”. The Hubble Space Telescope stared at a seemingly-empty, tiny patch of space for months; revealed were thousands of galaxies, most still in the process of coalescing at what was, for them, just 400 to 800 million years after the Big Bang (around 13 billion years ago for us).
Where would these galaxies be today? Because of expansion of the universe they have all been carried away from us so far that their current forms are not part of our visible universe.
Questions about the above images:
1. The blue-white region below-left of the Horsehead Nebula (image 2) is a ________ ?
(a) emission nebula
(b) over-exposed bright star
(c) planetary nebula
(d) reflection nebula
2. The Eagle Nebula (image 3) is likely to evolve into
(a) a single black hole.
(b) a single supergiant star.
(c) a star cluster with thousands of stars
Galaxies & Nebulae – 13
(d) nothing, it is dispersing gases.
3. The bright orange-red star in the Jewel Box (image 7) is which spectral type?
(a) A
(b) B
(c) M
(d) O
4. Not every picture in the planetary nebula montage (image 12) was a planetary nebula. You should recognize two imposters, name those two. _______________ and _______________
(a) Crab Nebula
(d) Horsehead Nebula
(b) Eagle Nebula
(e) SN 1987A
(c) Eta Carinae
(f) Triangulum Galaxy
5. If the Crab Nebula (image 13) is 6500 light-years away, the explosion we saw
in 1054 AD actually occurred 6500 years earlier. What year was that?
(a) 2196 BC
(c) 4254 BC
(e) 5446 BC
(b) 3779 BC
(d) 4392 BC
(f) 7554 BC
6. Look at the galaxy we said resembled the Milky Way (image 15), note how the spiral arms appear blue while the central “nucleus” appears yellow. How do you explain this?
(i) Why are the spiral arms blue?
(a) Lots of hydrogen gas
(b) Lots of young, blue stars
(c) Mainly blue-white white dwarfs
(d) Mainly hot gas
(i) Why is the nucleus yellow?
(a) Mainly older stars
(b) Mainly young stars
(c) Mostly hot gas
(d) Mostly hydrogen gas
Deep-Sky Objects
Launch the Voyager 4 program at csub.apporto.com. Click on buttons along the right edge of the window to turn off the display of planets and stars. Change “Altazimuth” to "Equatorial" using the pop-up menu at the bottom of the screen.
Select the Display/Deep Sky Objects… menu. Click on the "Magnitude Limits" button, drag the right control slider from its current value around 9 up to "all". Click OK. This causes the Voyager program to display all the "deep-sky" (very distant) objects it can. Click on the check box to select "Show Deep Sky Objects". We'll go with the default choices so just click OK once more.
What you are seeing:
Light Blue Ovals
........................
Spiral Galaxies
Yellow Ovals
.............................
Elliptical Galaxies
Dark Blue Ovals
.........................
Irregular Galaxies
White Ovals
...............................
Unclassified Galaxies
Yellow Crossed Circles
..............
Globular Clusters (large clusters of stars which are part of our
own Galaxy, much closer to us than other galaxies).
Pink Areas
..................................
Bright Nebulae (part of our Galaxy).
Blue Dotted Circles
....................
Open Clusters of stars (in our Galaxy).
Green Circles
.............................
Planetary Nebulae (in our Galaxy).
[Why do I suddenly feel like I'm in a Lucky Charms commercial?]
Galaxies & Nebulae – 14
Scroll around the sky, you will see that some parts of the sky appear to be densely packed with galaxies while others have few galaxies but lots of nebulae or clusters instead. Select the Display /The Milky Way… menu, click the "Show Milky Way" checkbox and the "Draw Outline" option for the Outer Milky Way then click OK.
The Milky Way is a galaxy containing over 200 billion stars packed into a big disk shape. Our Sun is inside that disk and so the rest of the galaxy surrounds us, appearing along a band in the sky. The Milky Way also contains lots of gas clouds (nebulae), on screen you should see lots of pink areas clustered along the Milky Way.
Globular Clusters of stars are also part of the Milky Way, but these mostly surround the disk of stars and nebulae. On screen, the globular clusters appear all over the sky, not as strongly concentrated in the Milky Way region.
The Milky Way
Open the Settings File called "Milky Way". The jagged outlined area stretching across the screen is the boundary of the Milky Way area in the sky. It surrounds us because we are inside of it. The galaxy contains lots of clouds (nebulae) which is why you see lots of nebulae within the Milky Way region.
Q. Why is the boundary so irregular?
A. The boundary is mostly based on how our view is blocked by nearby nebulae. Those clouds have irregular shapes which result in the irregular boundary.
All of the stars you see at night are fellow members of our Milky Way galaxy. Most are nearby neighbors of our Sun. We are surrounded by neighbor stars which is why they do not all appear in the directions towards the Milky Way.
7. Which is the brightest star in the sky (point the cursor at the star to make sure you know which name belongs to it)?
Open the Settings File named "Blue Giants", the display shows only type O and B stars (these are all either main-sequence or supergiant stars). These appear strongly clustered along a band in the sky. That band is the Milky Way. Blue Giant stars are all very luminous and we can see them even when they are very far away. The Milky Way is disk shaped (like a pancake) and we run out of stars when looking up or down (out of the disk) but not when looking into the disk. Because we are seeing so much further when looking at blue giants is why we see more when looking towards the Milky Way.
Galaxy Clusters
Open the Settings File called "Virgo Galaxy Cluster". You are looking at a rather large portion of the sky, bigger than the entire Big Dipper constellation. You are looking at the Virgo Cluster of Galaxies, an enormous “Supercluster”. Most of the galaxies displayed are around 60 million light-years away. Our own Milky Way Galaxy and Local Group of galaxies are part of the Virgo Cluster.
Here, for better visibility, spiral galaxies are shown in blue, elliptical galaxies in yellow, irregular galaxies in green, and unclassified as red. These galaxies do all have names or at least catalog numbers. Select the Display/Labels/Show Labels menu and then select the Display/Labels/
Deep Sky Objects menu.
Uh, okay, that was a mistake. Select the Display/Labels/Show Labels menu again to turn off Galaxies & Nebulae – 15
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labels. Make sure there are no Info Panels or other windows blocking your view of the screen.
The number of Spiral Galaxies shown on screen is about 491. Count the number of Elliptical, Irregular, and Unclassified galaxies displayed (do NOT scroll or zoom the screen) and fill in the following table. Select the Display/Deep Sky Objects… menu to turn off the display of Spiral Galaxies to aid your counting.
After all the counting, add up to get the total number of galaxies. Then divide the number of galaxies of each type by that total and multiply by 100% to get each type’s percentage of the total. As a check of your work, the total of all your percentages should 100 or very close to it.
Virgo Cluster:
Type
Color
Number Seen
Percentage (%)
Spiral
Blue
491
Elliptical
Yellow
Irregular
Green
Unclassified
Red
Total
8. Irregular galaxies are supposed to be the most common type of galaxy, why do we see so few? [Hint: Irregular galaxies are small.]
9. There are both Giant Elliptical galaxies and Dwarf Elliptical galaxies, which do you think we are seeing more of? [Hint: Dwarf Ellipticals are about as bright as Irregulars but are less common.]
Stars at Night
How many distinct stars are visible in the night sky from a dark location? Millions? Well, let's count and see. You don't want to count all the stars? Okay, we'll use a polling method to simplify things.
Select the File/Open Recent Settings/Startup menu. Select the Display/Magnitude Limits… menu, it probably says “Deep Sky Objects”, change that to “Stars”. Drag the slider along the left side to 6.2 – make sure it says 6.2 in the box. The right-side slider can be left at 6.0. Click OK.
The dimmest that can be seen without optical aids from locations with dark skies and no moonlight is 6.0 to 6.5, we’ll use 6.2 as a compromise value. Hide the green of the Earth/horizon by changing the pop-up menu at the bottom of the screen from Altazimuth to Equatorial. Use the scroll bars to move the sky view to any random direction. Zoom to 20°, closes the Time Panel, and count the stars on screen.
Too dim to see? Alright, select the Display/Stars… menu and move the "Dimmest Star" slider up. Click OK, your screen will now show dozens of colorful dots, like in the image at right. Count the stars on the screen, don't scroll, just count those currently visible, you can turn on grid lines as an aid to counting.
Galaxies & Nebulae – 16
Stars: __________
Scroll to some new, random patch of sky and count the stars again. Then do it a third time.
Stars: __________
Stars: __________
Just like a poll-taker may interview only a few people to project the total votes in an election, counting stars in a few random patches of the sky will allow us to estimate the total number of stars in the sky.
At the bottom of the screen, next to the zoom controls, is the current view size in degrees (it should say something like "20.0° x 15.0°"
). The total area of the sky displayed in the current chart window is calculated as the product of these two angles (e.g. 20° x 15° = 300 sd, where sd is short for square degrees).
Calculate your
viewing area and record it here:
Area = _________ sd
Average your three star counts and put that value here:
Average Stars = __________
The total area of the celestial sphere is [360
2
/π sd]. To scale up your result and get an estimate of the total number of stars visible from Earth with the unaided eye, do this calculation:
Nighttime stars = (Avg. Stars) / (Area) = ______________
[In case you the formula doesn’t display correctly or you’re just confused, here is what you do. Take your (Average Stars) number and multiply it by 360 and then multiply by 360 a second time, divide that result by pi (3.14) and divide that by the number you found for your (Area) above.]
Common estimates for the number of dark-sky stars vary from 6000 to 10,000; your estimate could fall outside that range (polls have margins of error). This is the number of stars for the whole celestial sphere, the Earth only allows you to see up to half the sky at a time and only with dark, clear skies. From Bakersfield, you'd be lucky to see a thousand stars even if it looks like millions.
A Trip to the Stars
Open the Settings File called "Flying Around Pleiades". The small (very small) cluster of stars at the very center of the screen is the Pleiades star cluster, also known as the "Seven Sisters". Click the Start button to take your imaginary journey to, around, and back from the Pleiades.
All those blue streaks? Those are constellation lines getting distored because you moved far from the Earth’s location.
10. How far is Alcyone from Earth? Hint: First, re-open the Settings File to put your location back at the Sun and Earth. Then, move the cursor over the star, click to get its Info Panel, and write down the distance (in ly) listed.
Galaxies & Nebulae – 17
Galaxies and Hubble's Law
Re-open the Settings File named "Virgo Galaxy Cluster". Zoom to 360° and scroll around. You may notice that galaxies all but vanish along a circular band, astronomers call this the "zone of avoidance".
It is not that galaxies actually avoid these locations, rather that is the region occupied by our Milky Way galaxy (and all its gas and dust) which blocks our view of galaxies in those directions (you may see many star clusters – dotted circles – along that band, those are the numerous open clusters of stars within our galaxy, many quite near to us).
Click on a few random galaxies and look under the Physical tab in their Info Panel. In many cases Voyager has values for the Radial Velocity, in both km/sec and as what percent that is of the speed of light. These are speeds that galaxies are moving away from us (or, very rarely, towards).
When astronomers take spectra of light from galaxies they see that all galaxies have the same pattern of absorption lines but shifted by various amounts towards longer wavelengths. This redshift can be measured and converted into a recessional velocity using the Doppler Shift formula.
By measuring the distance to these galaxies (maybe from the assumed size of the galaxy or maybe by using Cepheid variable stars), a correlation would be apparent. Almost all galaxies are moving away from us and galaxies that are further away are moving away faster.
Edwin Hubble discovered this correlation which can be expressed mathematically as
V
= H d
where
V
= recessional velocity (usually in km/s)
d
= distance away (usually in Mpc or Mly, megaparsecs or megalightyears)
H
= the proportionality constant between V
and d
, now called the Hubble constant,
= 72 km/s/Mpc = 22 km/s/Mly
For whatever galaxy you have currently selected, record its Radial Velocity value. For best results, if the radial velocity is less than 1000 km/sec, pick a different galaxy.
V
= _______________ km/sec
Now use Hubble's Law and the H
value above to calculate the distance to the galaxy in megalightyears (Mly). (Use the 22 value because that’s the one that will give us Mly units.)
d
= ______________ Mly
What distance is listed for the galaxy under the General tab?
d
= ______________ Mly
Let's try that again in reverse. Click on some other random galaxy and record the distance listed under the General tab in megaparsecs
.
d
= ______________ Mpc
Use Hubble's law to calculate the expected recessional velocity for this distance. (We’re using Mpc now, make sure you use the appropriate H
value.)
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V
= ______________ km/s
Now check the radial velocity given for that galaxy under the Physical tab.
V
= ______________ km/sec
Your answers may disagree slightly with Voyager's because Hubble's Law only predicts velocities due to the expansion of the universe, galaxies can have additional speeds due to their motion through space.
Expansion of the Universe
This behavior of galaxies (moving away from us and proportionately faster the further they are away) is explained by hypothesizing that we are in an expanding universe. Galaxies are being moved away from each other because the space between them is stretching larger; and galaxies further apart have more of that stretching space and move apart faster. [Galaxies near each other can easily move through space towards each other faster than that space is expanding.] That the universe is expanding is now an accepted part of modern cosmology.
Galaxies aren't really moving away from each other, they may not even be moving through space at all; space is carrying them away from each other. The redshift of the light from distant galaxies is the result of light waves being stretched longer as space expands during the light's long journey, not a Doppler shift due to that galaxy’s speed through space.
The Hubble constant measures the rate at which the universe expands, but written as 72 km/s/Mpc it looks almost incomprehensible. It can be re-written using simple unit conversions into:
H = (7% growth per billion years)
This form is more easily understood, it means that the universe is growing by 7% (a factor of 1.07) with each passing billion years.
11. If the distance between two stationary galaxies is 4.3 billion light years, how far apart will they be after (a) a billion years? (b) two billion years? (c) five billion years? [See the answers for hints.]
If we use Hubble's Law and calculate that a galaxy is 5 billion light years away, is that the distance to the galaxy now or the distance to the galaxy when it emitted the light?
Neither, it is the distance the light traveled. The calculations are tricky, but a result of D = 5 billion light years obtained using Hubble's Law corresponds to this situation:
• light emitted by that galaxy taking 5 billion years to reach us (and traveling a distance of 5 billion light years)
• that galaxy was 4.3 billion light years away from us when it emitted the light
• the galaxy is 6.0 billion light years away when we finally see it
Astronomers have recently learned that the rate of expansion is increasing, that modifies some of the calculations we did above. Until they can work out exactly how the expansion rate has Galaxies & Nebulae – 19
changed and will change, the origin and fate of the universe will be unknown.
Answers
1. Answer hidden
2. (c) a star cluster with thousands of stars
3. Answer hidden
4. (c) Eta Carinae; other answer hidden
5. Answer hidden
6. (i) (b) Lots of young, blue stars; (ii) Answer hidden
7. Sirius
8. Irregular galaxies, because of their small size, are not very bright (luminous). The view you have of the Virgo Cluster shows only the brightest galaxies. The Virgo Cluster probably contains a thousand (or more) irregulars but they are just too dim to be seen.
9. Answer hidden
10. Answer hidden
11. (a) 1.07 x (4.3 billion) = 4.60 billion light-years
(b) 1.07 x (1.07 x 4.3 billion) = 4.92 billion light-years
(c) Answer hidden
Galaxies & Nebulae – 20