If you speed up this movement, it would be possible to
see the Sun “wobble” back and forth in place as its planets
travel around it. Now, the Earth is a relatively small world
and has a small eect—we have been able to detect Earth-
sized planets, but at the moment it’s much easier to spot
so-called “super Jupiters.”
These massive gas giants make their stars wobble
dramatically. The ﬁrst planets we found outside the Solar
System were super-Jupiters, huge worlds that orbit very
close and very fast around their stars.
Over the last decade, our planet-hunting techniques
have improved, and we’ve launched better satellites into
orbit around Earth—a NASA telescope called Kepler is
one of the most important.
Kepler’s main job is to ﬁnd planets, and its instruments
are so sensitive it can even detect the shadow of a planet
passing across the front of a star—like our Moon making a
By seeing how the light from the star changes as the
planet passes—or “transits”—scientists can even ﬁgure
out what color the planet is.
There’s one planet with the rather unromantic name of
HD 189733b that is colored a bright blue, even bluer than
Earth. It’s not very Earth-like, though: it orbits close to its
star, one side is permanently dark, and on the surface it
rains molten glass.
Need something weirder? How about a planet so big it’s
almost a star? Or one with a surface made of diamond? Or
a super-Earth covered in an endless ocean?
We’ve only surveyed a fraction of the sky for planets
so far, but at the current rate of discovery, it’s looking
like there are hundreds of billions of worlds in our galaxy
wobbles in re-
sponse to planet’s
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
How much of the universe can I see with the
On a really dark night, far from the city, the sky is absolutely crammed with stars. How
much of the universe can we take in just lying on our backs in a ﬁeld?
The unaided human eye can see an inﬁnitesimally small portion of the universe—barely 3,000 stars and a
handful of other objects at a time. The whole picture is much bigger ….
Today, there are roughly 6,000 stars bright enough
for us to see while standing on the surface of
Earth. Add to those some of the gas clouds in the
Milky Way, a handful of other nebulae that show
up as pale smudges in the sky, the Small and Large
Magellanic Clouds that are nearby galaxies, and the
Andromeda galaxy if you know where to look.
Of those 6,000 stars, you can only ever see
around half of them at a time because the horizon
will block your view of the rest. If you wait patient-
ly, the rotation of the Earth will bring more of them
into view as the night passes.
Part of the reason we can only see 6,000
stars is because of light pollution. Even far away
from cities, the atmosphere reﬂects enough
light to wash out the faintest stars. Before
industrialization the night sky was quite a bit
darker, and humans may have been able to see
as many as 45,000 stars—though because of the
way the atmosphere absorbs starlight, it might
have been fewer.
The brightest star you can see in the North-
ern Hemisphere is Sirius, the Dog Star. In the
Southern Hemisphere, Alpha Centauri is both
the brightest star and also the closest to Earth
(technically the closest star is its smaller com-
panion, Proxima Centauri, but they’re so close
they look like one star).
Stars are very faint compared to the nor-
mal things we look at, and our eyes aren’t well
adapted to naked-eye astronomy. We have to
use the light-sensitive “rods” in our retinas
rather than the color-sensitive “cones,” so stars
mostly look white or greyish. If you really concentrate
you can pick out some stars that are redder or bluer than
others, but it’s tricky.
Starting with Galileo in the seventeenth century, hu-
mans developed telescopes to massively boost our ability
to see the universe. And when we started to hit the limit
on optical telescopes, we invented radio telescopes that
allow us to “see” through gas clouds and pick out extreme-
ly faint and distant objects.
It was the astronomer Edwin Hubble in the late 1920s
who ﬁrst realized there were other galaxies, and over the
last 100 years we’ve discovered the universe is much,
much bigger than we thought.
The current estimate is that the universe has
100 billion galaxies, each with 100 billion stars
in it. So the number of stars is … bear with us …
10,000,000,000,000,000,000,000. No, we don’t have names
for them all.
Meanwhile, the search for so-called exoplanets con-
tinues, and at the rate we’re ﬁnding them in orbit around
stars here in the Milky Way, it’s likely there are many more
planets in the universe than stars.
We’ve come a long way in our understanding of the
universe from those long, dark nights lying on the hillside
and tracing the shapes of mythical creatures, gods, and
heroes in the patterns of stars overhead. It’s likely we’ll
expand outward to colonize at least some of those stars.
Who knows—the next time you look up at the night sky,
you might see the sun of one of your future descendants.
The actual number of stars that can be seen with the
naked eye are about 6,000 (and only half that at any
time because the Earth’s horizon blocks the rest). But
scientists estimate that there are actually as many as
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Why do we use “light year” as a measure of distance?
One of the more confusing concepts in cosmology is the way we measure distance be-
tween stars, because we use a word for measuring time. Light year is a weird term, so why
do we use it?
The distances between stars and galaxies are so vast that our Earth-bound measuring systems are far too
small. The “light year” was invented so cosmologists could use smaller numbers! Except now they have a
new, better measurement ….
Sometime around the eighteenth century, astron-
omers began to get serious about measuring the
distances between various objects in space.
They’d already ﬁgured out how far the Earth
is from the Sun (92.96 million miles or about
149,668,992km) and decided that was too big a
number to have to write down all the time. So they
came up with the “astronomical unit” or AU. The
Earth is therefore 1 AU from the Sun. Jupiter is
5.2 AU from the Sun. Much easier than writing
483,370,198 miles (777,908,927km).
The next step was to measure the distance
to a nearby star. In 1838, a German astronomer
named Friedrich Bessel used a combination of
complicated lenses and even more complicated
math to ﬁgure out a star called 61 Cygni was
660,000 AU from the Sun.
Clearly, astronomers were about to run into
the same problem they’d had with miles. Stars
were millions of AU from the Sun, so a neater
measure was needed.
At this time, scientists were starting to real-
ize that light is the fastest-moving thing in the
universe. So it made sense to use some property
of light’s speed to measure distance. Bessel
decided that the distance light traveled in one
year would be a useful measure.
And so the term “light year” was coined.
Bessel said his star 61 Cygni was 10.3 light years
from Earth. Today, we know it’s 11.4 light years
away, but Bessel didn’t have any help from com-
puters, so his calculation is impressively close.
Just so you know, a light year is about 6 trillion
miles/9.5 trillion kilometers. That number is so huge
it’s practically meaningless. Think about it this way: the
Moon is about a light-second away from Earth (the dis-
tance light takes one second to travel) and Earth is about
eight light-minutes from the Sun. Our nearest neighbor
star, Proxima Centauri, is four light years away. The galaxy
is about 100,000 light years across. And our big galactic
neighbor Andromeda is 2.5 million light years away.
But wait, we’re not done here. Because this is cosmol-
ogy we’re talking about, nothing is ever simple. While a
light year is a great way of talking about interstellar dis-
tances without ﬁlling up the page with numbers, it’s hard
to match light years with actual observations of stars from
the surface of the Earth.
Toward the end of the nineteenth century, astronomers
started using a dierent measurement that was more
useful. As with most things in cosmology, explaining this
new measure involves a lot of math and triangles and
orbital speeds and things, but at the end of the day it has to
do with how the position of a star appears to change in the
sky based on where Earth is in our orbit.
The new measurement was named in 1913 by English
astronomer Herbert Hall Turner. He called it a “parsec.”
You might have heard this word in a certain famous
science-ﬁction movie, where a roguish space freighter
captain claims he had made “the Kessel Run in less than
A parsec is about 3.26 light years, but it’s better for
cosmologists because it’s more accurately deﬁned than a
light year. And understanding exactly how far objects are
away from Earth is vital in building our picture of what
the universe really looks like.
1 arc second or
1/3600 of a degree
1 parsec or 19.2 trillion miles
Astronomers need to measure very large distances. In popular science literature the light year is commonly used (1 light
year = 5.878625 trillion miles or the distance light travels in one year). Scientists however prefer to use the parsec as a
measure of long distances because it is more accurate and easier to calculate.
If the base of an imaginary, right angled triangle is the line from the Earth to the Sun and the other two sides intersect at
an angle of 1 arc second, then the point where they intersect is one parsec from the right angle.
unit or the