Naked Eye Observations: Crash Course Astronomy #2

Summary

In this episode of Crash Course Astronomy, Phil Plait explores naked-eye observations, revealing the celestial spectacle visible without instruments. He discusses the variable brightness of stars, their colors, and how ancient astronomers grouped them into constellations. Plait also addresses light pollution's impact on stargazing and the distinction between stars and planets, concluding with the significance of Polaris for navigation and the Earth's rotational effect on the night sky.

Takeaways

• 🌌 Astronomy with the naked eye is possible without any optical aid, revealing the night sky's natural splendor.
• 👀 Normal vision allows you to see thousands of stars, with the exact number depending on your sight quality and conditions.
• 💡 Stars vary in brightness due to their intrinsic luminosity and distance from Earth, with the brightest stars being either close or exceptionally luminous.
• 📚 The ancient Greek astronomer Hipparchus developed a system of magnitudes to rank stars by brightness, which is still in use today.
• 🔭 The Hubble Space Telescope has observed stars as faint as magnitude 31, far beyond the capability of the human eye.
• 🌈 The brightest stars exhibit color to the naked eye, with colors ranging from blue to red, depending on the star's temperature.
• 🔍 Constellations are patterns of stars named after familiar objects, with 88 officially recognized constellations today.
• 🌃 Light pollution significantly affects astronomical observations and the visibility of celestial objects, including the Milky Way.
• 🌟 Planets can be distinguished from stars because they do not twinkle due to their closer proximity and larger apparent size.
• 🌍 The apparent motion of stars across the sky is a reflection of the Earth's rotation, with stars making circular paths over the course of a night.
• 🌐 The celestial sphere's poles and equator influence the path and visibility of stars, with Polaris being a constant in the northern sky due to its proximity to the north celestial pole.

Q & A

What does 'naked eye' mean in the context of astronomy as described in the script?

In the context of astronomy, 'naked eye' refers to observing celestial objects without the aid of any optical instruments such as binoculars or telescopes, relying solely on the human eye.

How many stars can a person with normal vision see at any given time?

A person with normal vision can see a few thousand stars at any given time, with the total number being roughly between six to ten thousand, depending on the quality of one's sight.

What is the reason behind the variation in brightness of stars as observed from Earth?

The variation in brightness of stars is due to two main factors: the intrinsic physical brightness of the stars themselves, which can vary greatly, and their distance from Earth, with more distant stars appearing fainter.

Who is credited with creating the first star catalog and what system did he use to rank stars by brightness?

The ancient Greek astronomer Hipparchus is credited with creating the first star catalog and he used a system called magnitudes, ranking stars from 1st to 6th magnitude based on their brightness.

What is the difference in brightness between the brightest star visible to the naked eye and the faintest star ever seen with the Hubble Space Telescope?

The faintest star ever seen with the Hubble Space Telescope is about magnitude 31, which is approximately 10 billion times fainter than the faintest star visible to the naked eye.

Why do only the brightest stars appear to have color to the naked eye?

Only the brightest stars appear to have color to the naked eye because the color receptors in the human eye are not very light-sensitive, and only the light from the brightest stars is strong enough to trigger these receptors.

What is a constellation and how are they officially recognized today?

A constellation is a group of stars that form a recognizable pattern in the sky. Today, there are 88 official constellations, whose boundaries are carefully delineated on the celestial sphere.

Why do stars appear to move across the sky over the course of a night?

Stars appear to move across the sky because of the Earth's rotation on its axis. As the Earth spins once every day, it creates the illusion that the sky is a sphere wheeling around us.

What is the significance of Polaris, also known as the North Star, and why does it appear motionless?

Polaris, the North Star, is significant because it is located very close to the north celestial pole. Due to its proximity to the pole, it does not appear to rise or set and remains motionless in the sky, always pointing north.

Why is light pollution a problem for astronomical observations and what are some ways to mitigate it?

Light pollution is a problem for astronomical observations because it lights up the sky, making faint celestial objects more difficult to see. Mitigating light pollution involves using the right kind of light fixtures that direct light downward to the ground, as advocated by organizations like the International Dark-Sky Association.

How can one distinguish planets from stars when observing the night sky with the naked eye?

One can distinguish planets from stars by observing their twinkling. Stars twinkle due to the turbulence of the Earth's atmosphere distorting their light, whereas planets, being closer and appearing larger, do not twinkle as much due to less atmospheric distortion.

Outlines

00:00

🌌 Naked Eye Astronomy

This paragraph introduces the concept of naked eye astronomy, which is observing the sky without the aid of binoculars or telescopes. Phil Plait, the host, explains that even without optical instruments, one can discern a vast number of stars and learn much about the universe. The paragraph describes the experience of star-gazing in a dark location, the varying brightness of stars, and the factors influencing their visibility, such as their intrinsic brightness and distance from Earth. It also touches on the historical significance of star catalogs and the magnitude system developed by Hipparchus, which is still in use today.

05:04

🌟 Star Characteristics and Light Pollution

This section delves into the characteristics of stars as seen by the naked eye, including their colors and patterns that form constellations. It discusses how the brightest stars appear colored due to their high light intensity triggering the color receptors in the human eye. The paragraph also explains the concept of constellations, which are groups of stars named after familiar objects, and how they are officially recognized today. The impact of light pollution on astronomical observations and the efforts to reduce it are highlighted, emphasizing the importance of preserving the night sky for everyone's enjoyment and the well-being of nocturnal wildlife.

10:06

🌍 Celestial Motion and Planetary Observations

The final paragraph discusses the apparent motion of stars across the sky, which is actually a reflection of the Earth's rotation. It explains how this motion creates the illusion of a celestial sphere spinning around the observer. The paragraph introduces the concept of the celestial poles and equator, and how they relate to the observed motion of stars, with Polaris, the North Star, serving as a fixed point in the northern sky. It also mentions the visibility of different stars based on the observer's location on Earth and the unique phenomenon of planets not twinkling like stars due to their proximity and larger apparent size.

Keywords

💡Naked Eye Observations

Naked Eye Observations refers to the practice of observing celestial objects without the aid of optical instruments like telescopes or binoculars. In the context of the video, it is the method by which ancient astronomers studied the universe, and it is still a way for us to appreciate the night sky's beauty. The script emphasizes that even without sophisticated equipment, one can discern a great deal about the cosmos just by looking at it.

💡Magnitude

In astronomy, magnitude is a measure of the brightness of celestial objects as seen by an observer on Earth. The script mentions Hipparchus, who created a system of magnitudes to rank stars by their brightness, with the brightest stars being 1st magnitude and the faintest visible to the naked eye being 6th magnitude. Today, this system has been expanded to include much fainter objects, such as stars with a magnitude of 31 as seen by the Hubble Space Telescope.

💡Constellations

Constellations are patterns or groups of stars that form recognizable shapes in the sky, named after mythological figures, animals, or objects. The script discusses how ancient astronomers divided the sky into constellations, such as Orion and Delphinus, which are easily recognizable to the human eye. Today, there are 88 officially recognized constellations, and their boundaries are clearly defined.

💡Light Pollution

Light pollution is the excessive or misdirected artificial light that brightens the night sky, making it difficult to observe celestial objects. The script highlights the impact of light pollution on astronomical observations and the natural world, and how it can be mitigated by using better lighting fixtures that direct light downward.

💡Planets

Planets are celestial bodies that orbit a star, like our Sun. The script points out that some of the brightest objects in the night sky are not stars but planets, which do not twinkle like stars due to their proximity and larger apparent size. It also mentions the five naked-eye planets visible from Earth: Mercury, Venus, Mars, Jupiter, and Saturn.

💡Twinkling

Twinkling, or the apparent fluctuation in brightness and position of stars, is caused by the Earth's atmosphere distorting the light that reaches us. The script explains that planets do not twinkle as much as stars because their light is less affected by atmospheric turbulence due to their closer proximity and larger apparent size.

💡Polaris

Polaris, also known as the North Star, is a moderately bright star that is located very close to the north celestial pole. The script notes that Polaris does not appear to move throughout the night, making it a useful navigational tool for finding true north, especially in the Northern Hemisphere.

💡Celestial Sphere

The celestial sphere is an imaginary sphere with the Earth at its center, upon which all celestial objects appear to be projected. The script describes how the stars seem to move across the sky due to the Earth's rotation, creating the illusion of a spinning celestial sphere.

💡Celestial Equator

The celestial equator is an imaginary line that divides the celestial sphere into the northern and southern celestial hemispheres, analogous to the Earth's equator. The script explains that stars on the celestial equator make large circles in the sky, while those closer to the celestial poles make smaller circles or appear motionless.

💡Star Colors

Star colors are a result of their temperature, with different colors indicating different spectral classes. The script mentions that the brightest stars can be seen in colors such as blue (Vega), red (Betelgeuse), and yellow (Capella), but only the brightest stars trigger the color receptors in the human eye, making fainter stars appear white.

💡Hipparchus

Hipparchus was an ancient Greek astronomer who is credited with creating the first star catalog and the system of magnitudes. The script notes his contribution to the field of astronomy by establishing a method to rank stars by their brightness, a system that is still in use today.

Highlights

Introduction to Crash Course Astronomy, emphasizing the capability of naked-eye observations without the need for nudity or binoculars.

Explanation of the term 'naked eye' in astronomy, referring to observations made without the aid of optical instruments.

The vast number of stars visible to the naked eye, with an estimated range of six to ten thousand stars under ideal conditions.

The variation in star brightness due to differences in intrinsic brightness and distance from Earth.

The concept that the brightest stars in the sky are a mix of close and distant but highly luminous stars.

Hipparchus's contribution to astronomy through the creation of the first star catalog and the magnitude system.

The modern adaptation of Hipparchus's magnitude system, with the faintest stars observable by the Hubble Space Telescope at magnitude 31.

Sirius, the Dog Star, being the brightest star in the night sky, approximately 1000 times brighter than the faintest visible star.

The visibility of star colors to the naked eye, with only the brightest stars appearing in distinct colors like blue, red, orange, and yellow.

The formation of constellations by ancient astronomers, based on pattern recognition in the night sky.

The official recognition of 88 constellations with carefully delineated boundaries in modern astronomy.

The distinction between constellations and asterisms, such as the Big Dipper being part of Ursa Major.

The origin of many star names from Arabic, influenced by Persian astronomer Abd al-Rahman al-Sufi's translations.

The use of Greek letters and numerical designations for stars within constellations to categorize them by brightness.

The issue of light pollution affecting astronomical observations and its impact on nocturnal wildlife.

Efforts to reduce light pollution through the use of intelligent lighting and advocacy by organizations like the International Dark-Sky Association.

The phenomenon of celestial motion, with stars appearing to move across the sky due to the Earth's rotation.

The celestial sphere analogy, explaining the varying circular paths of stars based on their position relative to the celestial poles.

Polaris, the North Star, as a fixed point in the sky due to its proximity to the north celestial pole.

The dependence of visible stars on the observer's location on Earth, with different stars visible at different latitudes.

The distinction between stars and planets in the night sky, with planets appearing as non-twinkling points of light.

The production credits for Crash Course Astronomy, including the team behind the episode and PBS Digital Studios.

Transcripts

Hey, Phil Plait here. Welcome to episode 2 of Crash Course Astronomy: Naked Eye Observations.

Despite the salacious title, nudity is not required.

In fact, given that a lot of astronomical observations are done at night, you may want to bundle up.

[Theme Music] "One giant leap for mankind"

As it relates to astronomy, “naked eye” means no binoculars, no telescope.

Just you, your eyeballs, and a nice, dark site from which to view the heavens.

After all, that’s how we did astronomy for thousands of years,

and it’s actually pretty amazing what you can figure out about the Universe just by looking at it.

Imagine you’re somewhere far away from city lights, where you have an unobstructed view of the cloudless sky.

The Sun sets, and for a few minutes you just watch as the sky darkens.

Then, you notice a star appear in the east, just over a tree.

Then another, and another, and within an hour or so you are standing beneath an incredible display,

the sky spangled with stars.

What do you notice right away? First, there are a lot of stars.

People with normal vision can see a few thousand stars at any given time, and if you want a round number,

there are very roughly six to ten thousand stars in total that are bright enough to detect by eye alone,

depending on how good your sight is.

The next thing you’ll notice is that they’re not all the same brightness.

A handful are very bright, a few more are a bit fainter but still pretty bright, and so on.

The faintest stars you can see are the most abundant, vastly outnumbering the bright ones.

This is due to a combination of two effects.

One is that stars aren’t all the same intrinsic, physical brightness.

Some are dim bulbs, while others are monsters, blasting out as much light in one second as the Sun does in a day.

The second factor is that not all stars are the same distance from us.

The farther away a star is, the fainter it is.

Interestingly, of the two dozen or so brightest stars in the sky, half are bright because they’re close to Earth,

and half are much farther away but incredibly luminous, so they still appear bright to us.

This is a running theme in astronomy, and science in general.

Some effects you see have more than one cause.

Things aren’t always as simple as they seem.

The ancient Greek astronomer Hipparchus is generally credited for creating the first catalog of stars,

ranking them by brightness.

He came up with a system called magnitudes, where the brightest stars were 1st magnitude,

the next brightest were 2nd magnitude, down to 6th magnitude.

We still use a variation of this system today, thousands of years later.

The faintest stars ever seen (using Hubble Space Telescope) are about magnitude 31 –

the faintest star you can see with your eye is about 10 billion times brighter!

The brightest star in the night sky — called Sirius, the Dog Star —

is about 1000 times brighter than the faintest star you can see.

Let’s take a closer look at some of those bright stars, like, say, Vega.

Notice anything about it? Yeah, it looks blue. And Betelgeuse looks red.

Arcturus is orange, Capella yellow. Those stars really are those colors.

By eye, only the brightest stars seem have color, while the fainter ones all just look white.

That’s because the color receptors in your eye aren’t very light-sensitive,

and only the brightest stars can trigger them.

Another thing you’ll notice is that stars aren’t scattered evenly across the sky.

They form patterns, shapes.

This is mostly coincidence, but humans are pattern-recognizing animals,

so it’s totally understandable that ancient astronomers divided the skies up into constellations

(literally sets or groups of stars), and named them after familiar objects.

Orion is probably the most famous constellation;

it really does look like a person, arms raised up, and most civilizations saw it that way.

There’s also tiny Delphinus; it’s only 5 stars, but it’s easy to see it as a dolphin jumping out of the water.

And Scorpius, which isn’t hard to imagine as a venomous arthropod.

Others, well, not so much. Pisces is a fish? Yeah, OK. Cancer is a crab? If you say so.

Although they were rather arbitrarily defined in ancient times, today we recognize 88 official constellations,

and their boundaries are carefully delineated on the sky.

When we say a star is in the constellation of Ophiuchus,

it’s because the location of the star puts it inside that constellation’s boundaries.

Think of them like states in the US:

the state lines are decided upon by mutual agreement, and a city can be in one state or the other.

Mind you, not every group of stars makes a constellation.

The Big Dipper, for example, is only one part of the constellation of Ursa Major, the Big Bear.

The bowl of the dipper is the bear’s haunches, and the handle is its tail.

But! Bears don't have tails!

So astronomers might be great at pattern recognition, but they're terrible at zoology.

Most of the brightest stars have proper names, usually Arabic.

During the Dark Ages, when Europe wasn’t so scientifically minded,

it was the Persian astronomer Abd al-Rahman al-Sufi who translated ancient Greek astronomy texts into Arabic,

and those names have stuck with us ever since.

However there are a lot more stars than there are proper names, so astronomers use other designations for them.

The stars in any constellation are given Greek letters in order of their brightness,

so we have Alpha Orionis, the brightest star in Orion, then Beta, and so.

Of course, you run out of letters quickly, too, so most modern catalogs just use numbers;

it’s a lot harder to run out of those.

Of course, just seeing all those faint stars can be tough, which brings us to this week’s “Focus On.”

Light pollution is a serious problem for astronomers.

This is light from street lamps, shopping centers, or wherever,

where the light gets blasted up into the sky instead of toward the ground.

This lights the up the sky, making fainter objects much more difficult to see.

That’s why observatories tend to be built in remote areas, as far from cities as possible.

Trying to observe faint galaxies under bright sky conditions is like trying to listen to

someone 50 feet away whispering at you at a rock concert.

This affects the sky you see as well.

From within a big city, it's impossible to see the Milky Way,

the faint streak of across the sky that’s actually the combined light of billions of stars.

It gets washed out with even mild light pollution.

Your view of Orion probably looks like this:

When from a dark site it looks like this:

It’s not just people who are affected by this, either.

Light pollution affects the way nocturnal animals hunt, how insects breed,

and more, by disrupting their normal daily cycles.

Cutting back light pollution is mostly just a matter of using the right kind of light fixtures outside,

directing the light down to the ground.

A lot of towns have worked to use better lighting, and have met with success.

This is due in large part to groups like the International Dark-Sky Association, GLOBE at Night, The World at Night,

and many more, who advocate using more intelligent lighting, and to help preserve our night sky.

The sky belongs to everyone, and we should do what we can to make sure it’s the best possible sky we can see.

Even if you don’t have dark skies, there’s another thing you can notice when you look up.

If you look carefully, you might see that a couple of the brightest stars look different than the others.

They don’t twinkle! That’s because they aren’t stars, they’re planets.

Twinkling happens because the air over our heads is turbulent,

and as it blows past, it distorts the incoming light from stars,

making them appear to slightly shift position and brightness several times per second.

But planets are much closer to us, and appear bigger, so the distortion doesn’t affect them as much.

There are five naked eye planets (not counting Earth): Mercury, Venus, Mars, Jupiter, and Saturn.

Uranus is right on the edge of visibility, and people with keen eyesight might be able to spot it.

Venus is actually the third brightest natural object in the sky, after the Sun and Moon.

Jupiter and Mars are frequently brighter than the brightest stars, too.

If you stay outside for an hour or two, you’ll notice something else that’s pretty obvious:

the stars move, like the sky is a gigantic sphere wheeling around you over the course of the night.

In fact, that’s how the ancients thought of it.

If you could measure it, you’d find this celestial sphere spins once every day.

Stars toward the east are rising over the horizon, and stars in the west are setting,

making a big circle over the course of the night (and presumably, day).

This is really just a reflection of the Earth spinning, of course.

The Earth rotates once a day, and we’re stuck to it,

so it looks like the sky is spinning around us in the opposite direction.

There’s an interesting thing that happens because of this. Look at a spinning globe.

It rotates on an axis that goes through the poles, and halfway between them is the Equator.

If you stand on the Equator, you make a big circle around the center of the Earth over a day.

But if you move north or south, toward one pole or the other, that circle gets smaller.

When you stand on the pole, you don’t make a circle at all; you just spin around in the same spot.

It’s the same thing with the sky.

As the sky spins over us, just like with the Earth, it has two poles and an Equator.

A star on the celestial Equator makes a big circle around the sky, and stars to the north or south make smaller ones.

A star right on the celestial pole wouldn’t appear to move at all, and would just hang there,

like it was nailed to that spot, all night long.

And this is just what we see! Photographic time exposures show it best.

The motions of the stars show up as streaks.

The longer the exposure, the longer the streaks as the stars rise and set, making their circular arcs in the sky.

You can see stars near the celestial equator making their big circles.

And, by coincidence, there’s also a middling-bright star that sits very close to the north celestial pole.

That’s called Polaris, the north or pole star.

Because of that, it doesn’t appear to rise or set, and is always to the north, motionless.

It really is coincidence; there’s no southern pole star, unless you count Sigma Octans,

a dim bulb barley visible by eye that’s not all that close to the south pole of the sky anyway.

But even Polaris isn’t exactly on the pole -- it’s offset a teeny bit.

So it does make a circle in the sky, but one so small you’d never notice.

By eye, night after night, Polaris is the constant in the sky, always there, never moving.

Remember, the sky’s motion is a reflection of the Earth’s motion.

If you were standing on the north pole of the Earth, you’d see Polaris at the sky’s zenith

— that is, straight overhead — fixed and unmoving.

Stars on the celestial equator would appear to circle the horizon once per day.

But this also means that stars south of the celestial equator can’t be seen from the Earth’s north pole!

They’re always below the horizon.

So this in turn means that which stars you see depends on where you are on Earth.

At the north pole, you only see stars north of the celestial equator.

At the Earth’s south pole, you only see stars south of the celestial equator.

From Antarctica, Polaris is forever hidden from view.

Standing on the Earth’s equator, you’d see Polaris on the horizon to the north,

and Sigma Octans on the horizon to the south,

and over the course of the day the entire celestial sphere would spin around you;

every star in the sky is eventually visible.

While Polaris may be constant, not everything is.

Sometimes you just have to wait a while to notice.

And to that point, you’ll have to wait a while to find out what I mean by this,

because we’ll be covering that in next week’s episode.

Today we talked about what you can see on a clear dark night with just your eyes:

thousands of stars, some brighter than others, arranged into patterns called constellations.

Stars have colors, even if we can’t see them with our eyes alone, and they rise and set as the Earth spins.

You can see different stars depending on where you are on Earth,

and if you’re in the northern hemisphere, Polaris will always point you toward north.

Crash Course is produced in association with PBS Digital Studios.

This episode was written by me, Phil Plait.

The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.

It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics team is Thought Café.