Forty Ways to Know a Star: Using Stars to Understand Astronomy

introduction

Looking out at the night sky, the stars may appear as nothing more than faint pinpricks of light interrupting the darkness. When observed more carefully, differences begin to jump out, even to the unaided eye. We can find stars that are redder than others, stars that are brighter, and some that are more densely collected together. If we call on assistance from humanity’s many telescopes, these differences become even more stark.

Beginning with these differences in color and brightness, and aided by our ability to observe our very own star—the Sun—close up and carefully, we’ve built up an understanding of how the stars function. We now know where they form their energy, how that energy can escape their depths, and what their outermost layers look like. This knowledge means that the impacts of living near our own star go far beyond simply the warmth needed to host life.

The explosive and destructive ends of the lives of stars have been observed both by ancient civilizations and modern technologies. Through state-of-the-art observations and models of these cataclysms, we’ve learned how to understand what remains after these detonations, if anything, how the elements we see on Earth came to be, and the particular debt Earth’s gold and silver owes to the end of a star’s existence.

These foundations have allowed us to understand even more complicated stars. We’ve learned about stars that change in brightness over time, some of them so predictably that we’ve been able to use them as distance markers in a Universe that they themselves have proved to be much larger than we had thought. The stars have taught us about invisible components of our Universe, which turn out to vastly outnumber the visible portions, and their glow has taught us about how galaxies formed and changed over cosmic time.

All along humanity’s journey to understand the Universe, it is the stars that have led and lit the way. In this book, I introduce forty different ways to understand the stars, the galaxies they inhabit, and the cosmos they illuminate. I hope you enjoy the journey, and come away with a deeper connection to those faint pinpricks of light.

[ONE] Know a star…

as a light in the sky

To begin to know the stars, we can simply gaze up into the dark night. When the Sun sets, and your eyes adjust to the darkness, your vision can capture light that’s been traveling for hundreds to thousands of years to reach the Earth—the light from the stars.

In extremely dark sky conditions, about 9,100 stars are visible to the unassisted human eye, and in moonless, ideal conditions, it’s an impressive sight, with brilliant points of light peppering the entire sky above you.

Humans have used the stars for storytelling, navigating, and measuring the seasons as far back as records go. Oral histories confirm that stargazing is a fundamentally human venture. While most of us don’t navigate by the stars anymore, storytelling has remained embedded in the names of the constellations. These come to us from the ancient Greeks, though of course every civilization had their own set of constellations and stories. The constellations refer to the characters of many Greek myths: the hero Perseus, whose story involves the beheading of Medusa, the winged horse Pegasus, and the rescue of Andromeda from a sea monster, Cetus. Medusa is represented as the star Algol within the constellation of Perseus; Andromeda sits nearby Perseus. Pegasus is visible in the summer sky as the Great Square of Pegasus (it’s a big square of bright stars). Cetus has no bright stars in it, but it’s still present as a region in the sky (fig. 1.1).

In modern astronomy, the International Astronomical Union (IAU) agreed in 1922 to use a set of eighty-eight constellations covering the entire night sky. The boundaries between those constellations were published in 1930 by Eugène Delporte, with the approval of the IAU. Since then, those boundaries and constellations have provided an easy way to quickly indicate where a star is in the sky—it will always be within some known, standardized constellation.

Most people in the world live alongside city lights and other artificial sources of illumination, and as these lights become brighter, the faintest stars disappear from view. By a 2016 estimate, 14 percent of the global population, 20 percent of the EU, and 37 percent of the US population lives in such city-illuminated regions that the skies are never truly dark and the eyeball never fully switches into night-vision mode. Singapore is the most affected—100 percent of the population in Singapore will not see anything darker than twilight in their skies (fig. 1.2).

Unless you live in a major city, you’re likely to see somewhere between the 9,100 stars of the darkest skies and those of the most light-polluted cities. But the brighter the star, the rarer they are—there are only forty-five stars in the night sky brighter than Polaris, the north star. The Orion Nebula, by contrast, which masquerades as a star in the sword of Orion, is fainter. If you can see that object, then another 513 stars would be bright enough to be seen (though of course, some of them will be under the horizon).

There’s an increasing push to preserve the remaining truly dark places in the world as dark sky sites: locations people could visit to acquaint themselves with what the night sky would have looked like to every human prior to the advent of industrial lighting. They tend to be fairly remote, unfortunately—large cities can scatter their light much further afield than one might expect. In the US, a number of national parks are classified as International Dark Sky Places, and the skies there are usually spectacular.

Stars do appear to twinkle in the sky, especially if it’s a windy night. This twinkling isn’t something the light from the star is doing as it leaves its star. Instead, it’s the impact of the Earth’s atmosphere. As you go from sea level up to the upper atmosphere, the temperature of the atmosphere drops dramatically, but if you look carefully, there are little pockets of warmer and cooler air, instead of a perfectly smooth gradient. Each little pocket can bend the incoming beam of light from the star, and so by the time the light makes it down to us, observing from the surface of the Earth, the dot of light can seem to flicker in brightness as our atmosphere blurs it in and out of focus. Since the beam of light from a star is so thin and narrow, even small disturbances become visible to our eyes (fig. 1.3).

The more pockets of air there are, the larger this impact is, and the easiest way to get a lot of small pockets of air is for the atmosphere to be windy. It doesn’t have to be windy at ground level, and it often isn’t, but if you see the stars twinkling and the air is still where you are, then you can have a pretty good guess that high above you, the atmosphere is blustery.

[TWO] Know a star…

as the source of daylight

Our own daytime hours are an easy way to appreciate the stars. For all the beauty and fantastical diversity of the night sky, there is one star in particular that so vastly outshines the rest of them that when it’s above our horizon, it floods the sky with so much light that seeing other stars is functionally impossible: our Sun.

The Sun is not bright in our skies because it’s particularly brighter than the rest of the stars, but simply because we’re so close to it. Close is an astrophysically relative term here—the distance between the Earth and the Sun is still a mind-boggling 93 million miles (150 million kilometers). But compared to the next closest star, Proxima Centauri, which sits 268,770 times further, we’re positively cozy with our Sun.

With a specially protected telescope, the Sun is an easy target to observe, so long as it’s not cloudy. From the ground, we can already begin to see things beyond a simple bright disk in the sky. Observations of the Sun well predate the telescope, with written records going back to 800 BCE in China. The first known drawing of a sunspot comes from 1128 CE, in John of Worcester’s Chronicle. Sunspots are dark patches on the Sun, where the surface is a bit cooler than the surrounding area. They arise whenever the magnetic field of the Sun becomes tangled and pops a loop out of the surface.

The tangling of the magnetic field happens because the equator of the Sun rotates faster than the poles—something we learned by watching how fast it takes a sunspot to cross the disk of the Sun. The more times the equator laps the poles, the more complex the magnetic field becomes, the more loops it’s likely to create, and the more likely it is for sunspots to appear on its surface (fig. 2.1).

Without technology to help, and under normal circumstances, sunspots are the only major defect in the Sun that can be seen with the human eye. When the magnetic field of the Sun is calm, observed carefully, our star looks like a featureless bright disk. However, with technological advances, we’ve been able to see the dynamic and ever changing features on the surface of the Sun. Ground-based telescopes have been able to image the Sun in enormous detail, showing a roiling surface with bubbles of intensely heated plasma rising in a constantly shifting glitter known as granulation. These bubbles are bounded by slightly cooler plasma, sinking back down into the depths of the star, in a process most akin to boiling water. The smallest granulation cells are roughly the size of Texas, with larger ones spanning a surface area that would cover the majority of the US.

We have also launched telescopes out into space to observe the Sun, without the hindrance of being offline for half of every day. Two of the most productive of these are the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO), both of which have returned a number of impressive images capturing detailed views of the Sun. SOHO has been placed in an orbit with an unobscured view of the Sun, so that the satellite is in permanent daylight. SDO orbits the Earth, but in a wide orbit where its view of the Sun is only occasionally blocked by our planet. SDO and SOHO combine to give us particularly good vistas of any dramatic changes on the solar surface, such as solar prominences. These are large loops of plasma, carried high above the surface along magnetic currents, many tens of times larger than the Earth. A prominence can persist over days to months, and during that time, the plasma caught up in the magnetic loop can gradually rain back