The Earth
The Earth- the home to over 6 billion humans and countless other life forms. The Earth has been the place where great achievements and grand failures have occurred. So what exactly is this thing we call “Earth”? The Earth is the third planet away from the Sun, as well as the largest of the terrestrial planets, which includes Mercury, Venus, and Mars in its ranks. The Earth is believed to have been formed about 4.6 billion years ago. The Earth is unique in that it is the only planet in our solar system, and possibly the universe, that contains life. In order to support this life, our planet has a huge supply of water called “oceans” that covers approximately 70% of the surface. The rest of the land is distributed in the land masses we call “continents”.
The Earth goes around our Sun once every 365 or so days (it goes around approximately 365.25, so this causes us to compensate with leap years in which we add an extra day). It orbits the Sun in an ellipsis, which looks like a stretched out circle. Because of this, there is a point where every year, it gets the closest possible to the Sun, which is called perihelion. There is also a point where it gets the farthest possible in its orbit, called aphelion. The Earth spins on its axis, an imaginary line going from the North to the South Pole. This line isn’t completely straight, which is because the Earth spins on an angle of about 23.5 degrees. This is the reason for the creation of seasons- the part that is angled towards the Sun experiences the warmer weather than the other part.
The Earth is made up of rocky material, and has several layers to it, almost like an onion. You are currently residing on its surface, or the crust. The layer under that is the mantle, which is like molten rock. Mantle material comes up occasionally to the surface in the form of magma. After the mantle (which, by the way, is a big layer), you will reach the outer core, a region of liquid heavy metals. The innermost layer is the inner core, which is a solid chunk of iron and nickel. This inner core spins, and because of it, Earth has a magnetic field, which is useful when you’re lost and the only thing to guide you is a compass.
Going in the opposite direction is the atmosphere, the air you and I breathe. This also has layers, although they are not as visibly distinguishable. The lowest layer in which we breathe in every day is the troposphere. This is where all your weather happens. The layer above it is the stratosphere, which apart from having a cool name, holds the ozone layer, which is vital in blocking out harmful ultraviolet rays from reaching us. The layer above that is the mesosphere, which is where most of the meteors you see at night burn up and disintegrate. The thermosphere is still further upward, and the ions it contains due to the solar wind is what makes our radio systems reach to faraway places, as well as house the famous auroras. Finally, the highest layer is the exosphere, which is the final frontier before we reach the coldness of space.
The Milky Way
The Milky Way is our Galaxy, the system of stars, gas, dust, and other objects in which our solar system lives. Its name comes from the whitish swath across the sky that looked to ancient observers like spilled milk. Nowadays, the Milky Way is harder to spot in many areas because of light pollution, the excess from city lights that makes the sky glow — often yellowish or orange-ish — and washes out the stars. If you live in a city, you may well have never seen the Milky Way. If you haven’t, make a point of going out to a dark place and looking up!
The Milky Way has four main components: a
disk, bulge, halo, and dark matter.
The swath we see in the sky is the disk. It contains over 100 billion stars, along with gas and dust from the interstellar medium, the gas and dust between the stars. The ISM provides the material for star formation, and dying stars replenish it when they throw off their outer layers at death. The disk is 100,000 light-years across — that is, it takes a particle of light, or photon, 100,000 years to cross from one end of the Milky Way to the other.
The disk has spiral arms, concentrations of gas and dust created where material is compressed. While a permanent feature, the arms do not always contain the same stars — if they did, the arms would wind up after a few million years, like thread on a spinning spool. Instead, stars and ISM actually travel through the arms on their roughly circular orbits around the Galaxy’s center. The arms are places where the material has been pressed together, triggering star formation by compressing gas clouds to such high pressures and temperatures that nuclear fusion starts, making those regions look brighter.
Sounds bizarre, right? If you have trouble visualizing this process, think of a busy highway with a big truck moving more slowly than the other cars:
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The other cars have to slow down while they’re near the truck, but the same cars don’t stay with the truck the whole time. They move past the truck and speed up again. But because the truck is still there, there will always be a knot of cars moving along the highway passing the truck — just not the same cars.
The same idea holds for the Galaxy’s density waves, the areas of compression the stars and gas pass through: there will always be stars in the arm, but they’re not always the same stars. And since the Galaxy’s most massive and brightest stars form in these compressed regions, the pattern stands out because these stars are brighter than the others in the Milky Way. These stars die before they move too far beyond the pattern, because more massive stars live shorter lives, and so the stars outside the arms are fainter and less massive leftovers.
The
Other Stuff
The bulge and halo have older stars that
orbit in less orderly fashions than those in the disk. There’s
little gas or dust in these regions, if any. The bulge is not to
be confused with the Galactic nucleus, although it does surround
that region. The Galactic nucleus is a small region at the Milky
Way’s center with a high concentration of fast-moving stars,
orbiting what astronomers believe is a supermassive black hole —
that is, a black hole at least 2 million times the Sun’s mass.
And what about dark matter? Astronomers know dark matter exists in the Milky Way for a couple of reasons. One is the speed of material moving through the disk. According to Kepler’s laws of orbital motion, the speed a star orbits around the Galactic center falls off at further and further distances from the center. The exact speed depends on how much mass sits within the star’s orbit. If the only matter in the Galaxy were what we see, then star and cloud speeds far out in the disk should fall off rapidly. They don’t. Instead, they rise slightly and stay roughly constant for much longer than astronomers expect. The only explanation for this rotational velocity curve is the presence of additional, unseen matter, which astronomers call dark matter.
Dark matter is like normal matter in that it has mass, so it affects things around it gravitationally. It is unlike normal matter, however, because it doesn’t interact with anything (even light) through collisions, absorption, or emission. Dark matter is made up of particles, but no one knows exactly what these particles are like yet.
The Milky Way’s Formation, In Brief
Astronomers’ understanding of Galactic
formation has evolved in recent decades. Originally, astronomers
imagined the Galaxy formed from a single, galactic-scale cloud
of baryonic matter — i.e. matter we can interact with, like
protons (I called it “normal matter” before) — that collapsed in
on itself due to its own self-gravity.
More recent models take a “bottom-up”
approach to galaxy building. In the hierarchical scenario,
the first conglomerations of matter were about 106 Msolar
(today’s galaxies are around 1011) and
congregated in wells of dark matter. Dark matter coalesced
early-on into dense regions because, since it is
non-interacting, it does not feel the effects of interior
pressure from particles’ kinetic (thermal) energy. Baryons, on
the other hand, cannot clump while hot. They had to wait until
photons decoupled from matter (i.e. quit interacting with
energetic free electrons in the early Universe, which was hot)
to cool enough to form clouds. It’s a simplification, but if you
think of a huge blob of plasma with crazy collisions and
particles bouncing around inside it, you’ll have the gist.
Once things cooled down enough, baryons could fall into the gravitational wells (the sinks in space-time created by mass) dark matter had already created. Without dark matter, baryons just couldn’t collapse together fast enough to explain the galaxies and clusters we see today. And studies show that the 106 solar mass parameter I mentioned earlier is the mass at which clouds start to collapse in on themselves, forming stars. Observations of the early Universe also suggest objects this size may have served as galactic building blocks, coming together to form the larger galaxies we see today.
Anyway, the short of it is that clouds started clumping together into one cloud, which collapsed in on itself. As it collapsed, the cloud formed stars (the halo) by compressing gas enough to kickstart nuclear fusion. The gas particles collapsing in the polar directions collided and cancelled out each other’s motions. The particles cooled off, losing energy. The loss of energy is important in explaining the disk: the circle is the lowest orbital energy level — that is, a particle, gas cloud, or star has to have more kinetic energy if it wants to move in, say, the non-circular orbits stars inhabit in the Galactic halo, which are randomly inclined to the disk’s plane. So the gas in the disk lost a lot of energy through collisions and radiating off photons, and it settled into the disk we see today.