by Kaitlin Muro & NASA
2
contents chapter 1
the moon
Chapter 2
stars and nebulas
chapter 3
galaxies
3
4
5
6
solar eclipse As seen from the Earth, a solar eclipse is a type of eclipse that occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks (“occults�) the Sun. This can happen only at new moon, when the Sun and the Moon are in conjunction as seen from Earth in an alignment referred to as syzygy. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses only part of the Sun is obscured.
7
8
If the Moon were in a perfectly circular
Nevertheless, in some ancient and
orbit, a little closer to the Earth, and in
modern cultures, solar eclipses have
the same orbital plane, there would be
been attributed to supernatural causes
total solar eclipses every single month.
or regarded as bad omens. A total solar
However, the Moon’s orbit is inclined
eclipse can be frightening to people
at more than 5 degrees to Earth’s orbit
who are unaware of its astronomical
around the Sun (see ecliptic) so its
explanation, as the Sun seems to
shadow at new moon usually misses
disappear during the day and the sky
Earth.
darkens in a matter of minutes.
Earth’s orbit is called the ecliptic plane
Since looking directly at the Sun
as the Moon’s orbit must cross this plane
can lead to permanent eye damage
in order for an eclipse (both solar as well
or blindness, special eye protection or
as lunar) to occur. In addition, the Moon’s
indirect viewing techniques are used when
actual orbit is elliptical, often taking it far
viewing a solar eclipse. It is technically
enough away from Earth that its apparent
safe to view only the total phase of a
size is not large enough to block the Sun
total solar eclipse with the unaided eye
totally. The orbital planes cross each year
and without protection, however this is
at a line of nodes resulting in at least two,
a dangerous practice as most people
and up to five, solar eclipses occurring
are not trained to recognize the phases
each year; no more than two of which
of an eclipse which can span over two
can be total eclipses. However, total solar
hours while the total phase can only last
eclipses are rare at any particular location
up to 7.5 minutes for any one location.
because totality exists only along a
People referred to as eclipse chasers or
narrow path on Earth’s surface traced by
umbraphiles will travel to remote locations
the Moon’s shadow or umbra.
to observe or witness predicted central
solar eclipses.
An eclipse is a natural phenomenon.
9
10
Created from exposures taken at intervals of 8.5 minutes during the total eclipse phase, the midpoint of the eclipse corresponds to the central exposure. The play of light across the lunar surface nicely demonstrates that the Earth’s shadow is not uniformly dark as it extends into space. In fact, lunar maria and montes are still visible in the dimmed, reddened sunlight scattered into the cone-shaped shadow region, or umbra, by the atmosphere. Still, while processing the pictures into this composite image, astronomer Sebastien Gauthier was reminded of another haunting orange face.
11
12
The dark side of the moon is a myth. In reality both sides of the Moon see the same amount of Sun however only one side of the Moon is ever seen from Earth this is because the Moon rotates around on its own axis in exactly the same time it takes to orbit the Earth, meaning the same side is always facing the Earth. The side we see is lit by reflected sunlight, while the side facing away from Earth lies in darkness and has only been seen by the human eye from a spacecraft. The rise and fall of the tides on Earth is caused by the Moon. There are two bulges in the Earth due to the gravitational pull that the Moon exerts; one on the side facing the Moon, and the other on the opposite side that faces away from the Moon, The bulges move around the oceans as the Earth rotates, causing high and low tides around the globe. The Moon is drifting away from the Earth. The Moon is moving approximately 3.8 cm away from our planet every year. It is estimated that it will continue to do so for around 50 billion years.
By the time that happens, the Moon will be taking around 47 days to orbit the Earth instead of the current 27.3 days. A person would weigh much less on the Moon. The Moon has much weaker gravity than Earth, due to its smaller mass, so you would weigh about one sixth (16.5%) of your weight on Earth. This is why the lunar astronauts could leap and bound so high in the air. The Moon has only been walked on by 12 people; all American males. The first man to set foot on the Moon in 1969 was Neil Armstrong on the Apollo 11 mission, while the last man to walk on the Moon in 1972 was Gene Cernan on the Apollo 17 mission. Since then the Moon has only be visited by unmanned vehicles. The Moon has no atmosphere. This means that the surface of the Moon is unprotected from cosmic rays, meteorites and solar winds, and has huge temperature variations. The lack of atmosphere means no sound can be heard on the Moon, and the sky always appears black.
13
14
15
16
STars Look up in the night sky and you’ll see lots of stars. But what is a star? In a scientific sense, a star is ball of hydrogen and helium with enough mass that it can sustain nuclear fusion at its core. Our Sun is a star, of course, but they can come in different sizes and colors. Seventy-five percent of the matter in the Universe is hydrogen and twenty-three percent is helium; these are the amounts left over from the Big Bang. These elements exist in large stable clouds of cold molecular gas. At some point a gravitational disturbance, like a supernova explosion or a galaxy collision will cause a cloud of gas to collapse, beginning the process of star formation.
17
As the gas collects together, it heats up. Conservation of momentum from the movement of all the particles in the cloud causes the whole cloud to begin spinning. Most of the mass collects in the center, but the rapid rotation of the cloud causes it to flatten out into a protoplanetary disk. It’s out of this disk that planets will eventually form, but that’s another story. The protostar at the heart of the cloud heats up from the gravitational collapse of all the hydrogen and helium, and over the course of about 100,000 years, it gets hotter and hotter becoming a T Tauri star. Finally after about 100 million years of collapse, temperatures and pressures at its core become sufficient that nuclear fusion can ignite. From this point on, the object is a star. Nuclear fusion is what defines a star, but they can vary in mass. And the different amounts of mass give a star its properties. The least massive star
18
possible is about 75 times the mass of Jupiter. In other words, if you could find 74 more Jupiters and mash them together, you’d get a star. The most massive star possible is still an issue of scientific disagreement, but it’s thought to be about 150 times the mass of the Sun. More than that, and the star just can’t hold itself together. The least massive stars are red dwarf stars, and will consume small amounts over tremendous periods of time. Astronomers have calculated that there are red dwarf stars that could live 10 trillion years. They put out a fraction of the energy released by the Sun. The largest supergiant stars, on the other hand, have very short lives. A star like Eta Carinae, with 150 times the mass of the Sun is emitting more than 1 million times as much energy as the Sun. It has probably only lasted a few million years and will soon detonate as a powerful supernova; destroying itself completely.
19
thor’s helmet
20
nebulas A nebula is an interstellar cloud in outer space that is made up of dust, hydrogen and helium gas, and plasma. It is formed when portions of the interstellar medium collapse and clump together due to the gravitational attraction of the particles that comprise them. Nebulae, like Thor’s Helmet over there, are often favorite astronomical objects of scientists who want to learn more about stellar or planetary formation. You see, parts of a nebula may clump together some more. The gravitational forces between particles is directly proportional to the their masses.
21
Nebula is latin for ‘cloud’ so it is the word astronomers use to call huge clouds in space. Note that historically it was used to describe any fuzzylooking thing in the sky so older books, from say pre-1960, might call a galaxy nebula too but this isn’t done now. Nebulae come in several types. Giant Molecular Clouds are the most common but least noticed because they don’t look very exciting, being big, dark and cold clouds of dust and gas. An example would be the Taurus Molecular Cloud. However some GMCs can become star-forming regions. These are quite literally stellar nurseries. In these cases the energy from new stars lights up the surrounding gases and they can look spectacular. Just take a look at the Carina Nebula (NGC 3372), if it wasn’t for all the new stars there it would be a murky dark mass, but just add their radiation and you get this amazing expanse of light and colour.
22
Can we see any? The Orion nebula is a Vvery famous nebula and we know that it is full of stars. The stars form deep inside the nebula then as they light up radiation and particles flowing out of them blow away the surrounding gas and dust. Looking deep into the Orion Nebula with the Hubble Space Telescope reveals that the material is churning around in wonderful wispy patterns. This process will eventually disperse the nebula but in the course of it the nebula gets sculpted into weird shapes, like fingers, fairytale castles and so on. This is happening a bit in the Orion Nebula but is really noticeable in another famous one, the Eagle Nebula in the summer constellation of Serpens. There are some extraordinary spires of dust and gas there. The Carina Nebula isn’t as well-known but there are some lovely structures there. All of these are transitory features which will one day be gone.
23
24
25
26
galaxy formation There are two leading theories to explain how the first galaxies formed. The truth may involve a bit of both ideas. One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form. The other, which has gained strength in recent years, says the young universe contained many small “lumps� of matter, which clumped together to form galaxies. Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form ellipticals.
27
28
We really don’t know how various galaxies formed and took the many shapes that we see today. But we do have some ideas about their origins and evolution. Shortly after the big bang about 14 billion years ago, collapsing gas and dust clouds might have lead to the formation of galaxies. Interactions between galaxies, specifically collisions between galaxies, play an important role in their evolution. Let’s look at the period of galaxy formation. Edwin Hubble’s observations, and subsequent Hubble Law (which we’ll explain later), led to the idea that the universe is expanding. We can estimate the age of the universe based on the rate of expansion. Because some galaxies are billions of light years away from us, we can discern that they formed fairly soon after the big bang (as you look deeper into space, you see further back in time). Most galaxies formed early, but datafrom NASA’s Galaxy Explorer (GALEX) telescope indicate that
some new galaxies have formed relatively recently -- within the past few billion years. Most theories about the early universe make two assumptions: it was filled with hydrogen and helium or some areas were slightly denser than others. From these assumptions, astronomers believe that the denser areas slowed the expansion slightly, allowing gas to accumulate in small protogalactic clouds. In these clouds, gravity caused the gas and dust to collapse and form stars. These stars burned out quickly and became globular clusters, but gravity continued to collapse the clouds. As the clouds collapsed, they formed rotating disks. The rotating disks attracted more gas and dust with gravity and formed galactic disks. Inside the galactic disk, new stars formed. What remained on the outskirts of the original cloud were globular clusters and the halo composed of gas, dust and dark matter.
29
30
close to home The Andromeda galaxy, our Milky Way’s closest neighbor, is the most distant object in the sky that you can see with your unaided eye — but only on a clear night from a location with a very dark sky. The galaxy is a beautiful sprial, but one fact you may not be aware of: We’re safe for a few billion years, but Andromeda is headed our way and on a collision course with the Milky Way. Here we explain that cosmic train wreck, plus explain Andromeda’s location, shape and other facts.
31
32
Located just to the north of the constellation bearing its name, the Andromeda galaxy appears as a long, hazy patch in the sky. It should appear as a smudge in the sky, even with moderate light pollution. If you live in a more populated place, you may have more trouble. Binoculars will clearly reveal its shape. The visible fuzzy patch of stars stretches about as long as the width of the full moon, and half as wide; only with significant magnification can you tell it stretches six times that length in fullness. A spiral galaxy like the Milky Way, Andromeda contains a concentrated bulge of matter in the middle, surrounded by a disk of gas, dust, and stars 260,000 light-years long, more than 2.5 times as long as the Milky Way. Though Andromeda contains approximately a trillion stars to the quarter to half a billion in the Milky Way, our galaxy is actually more massive, because it is thought to contain more dark matter. Amazingly, this stretch of stars, which in our sky appears about as long as the full moon and half as wide, lies 2.5 million lightyears away, further than any star you can see with your eyes. Also known as M31, it is
the closest galaxy to the Milky Way - and it’s moving closer every day. Andromeda’s proximity will be deadly to our galaxy. The two galaxies are rushing closer to one another at about 70 miles per second (112 kilometers per second). Astronomers estimate that it will collide with the Milky Way in about 5 billion years. By that time, the sun will have swollen into a red giant and swallowed up the terrestrial planets, so Earth will have other things to worry about. Still, the fresh influx of dust should boost star formation in the new Milkomeda galaxy, and the Earthless sun may well leave the Milky Way for good. After a messy phase, where arms project crazily from the combined pair, the two should settle into a smooth elliptical galaxy. Galaxy collisions are a normal part of the universe’s evolution. In fact, both Andromeda and the Milky Way bear signs of having already crashed into other galaxies. Andromeda boasts a large ring of dust in its center, giving it an interesting shape. Astronomers believe this dust may have formed when it swallowed an existing galaxy.
33
“The dreams of yesterday are the hopes of today and the reality of tomorrow. The regret on our side is, they used to say years ago, we are reading about you in science class. Now they say, we are reading about you in history class. Curious that we spend more time congratulating people who have succeeded than encouraging people who have not.” -New York Times “Buy why, some say, the moon? Why choose this as our goal? And they may as well ask why climb the highest mountain?” -National Geographic