Notes

science Beyond Earth

This model for the solar system is called a geocentric model. The model is intuitive, in that from our point of view, it does seem that the sun, moon, and stars seem to revolve around us. And Roman cultural dominance over a long period of time instilled faith in the geocentric idea.

Gravity is defined as a force that pulls objects toward each other. Specifically, the sun's gravity keeps planets in our solar system in orbit around the sun.

Over the next 150 years, astronomical science flourished, culminating in 1687 with the publication of Isaac Newton's Principia. This work expounded on the laws of gravity and would form the foundation of physics for the next 200 years.

This became clearer once the German astronomer Johann Kepler (1571–1631) devised what became known as the laws of planetary motion. Kepler's work was built on an exhaustive analysis of astronomical data that had been collected by a Danish astronomer named Tycho Brahe (1546–1601). Brahe's observations were precise and careful enough for Kepler to build upon.

Kepler's first law, published in 1609, is sometimes called the orbit law. It states that all orbits are specific types of conic sections. Conic sections describe curves that can be created by slicing through a cone. There are four types of conic sections: the circle, the ellipse, the parabola, and the hyperbola. The parabola and hyperbola are both open curves, which simply means—in this context—that they represent orbits in which the object will not come back to the same position repeatedly. Comets are examples of bodies that have open orbits. Circles and ellipses are examples of closed orbits, meaning that the orbiting object will repeat the same path over and over. It's important to note that, from a geometric perspective, a circle is just a special type of ellipse, resulting from slicing a cone in a plane that's perpendicular to its base. The orbits of all the planets in the solar system are slightly elliptical.

The minor axis is drawn through the center of the ellipse at its narrowest point. The major axis is drawn through the center across its longest width, and travels through the two foci. It is related to the amount of time required for a planet to orbit completely around its star.

An ellipse has a focus at two points that lie along the major axis, called foci (foci is just the plural of focus). When a planet orbits a star, one focus point is within the star, and is what we think of as the point that the planet orbits around.

Kepler's second law, sometimes called the area law, states, "A line that connects a planet to the sun sweeps out equal areas in equal times." While this may seem complex, the law makes a statement about the amount of time required for a planet to move through a portion of its orbit. Consider, as an example, a planet moving in an elliptical orbit around a star. The planet moves faster when it's closer to the star but slower when it's farther away.

A planet's period, or revolution, is the amount of time it takes to make one complete orbit around the sun. For Earth, this time amount is 365.25 days, or one Earth year.

The semi-major axis is a section of the major axis that's half its total distance, or the distance from its center to its outer point. Kepler found that the square of the orbital period is proportional to the cube of the semi-major axis of the orbit.

A binary star system is a pair of stars with similar masses orbiting a shared point in space that's at the center of their combined masses. In our solar system, each of the planets exerts its own influence on the position of the sun, and the effects from all eight major planets partially cancel each other.

Solstices occur when earth’s axis is tilted directly toward and away from the sun. One hemisphere is having a summer solstice, the day of the year with the longest amount of sunlight and first day of summer. The other hemisphere is having their winter solstice, the day of the year with the shortest amount of sunlight, and first day of winter, at the same time. Halfway between the solstices are equinoxes, days that have approximately equal daylight and night. One equinox occurs in September, and in the Northern Hemisphere, this autumnal equinox marks the end of summer and the beginning of autumn. At this point, the days start getting shorter and the nights, longer. The opposite occurs on March 21, which is the spring equinox in the Northern Hemisphere: The days start getting longer, and the nights start getting shorter.

As the earth-moon system moves around the sun, the three bodies periodically align to produce eclipses. Two types of eclipses can occur. The first, called a solar eclipse, occurs when the moon passes between the sun and the earth, momentarily blocking the sun's rays from reaching certain areas of the earth.

The umbra is a darker shadow that lies directly behind the moon, while the penumbra is a lighter shadow that surrounds the umbra.

The second type of eclipse, called a lunar eclipse, occurs when the moon moves behind the earth. As in a solar eclipse, the earth can block sunlight from reaching the moon, creating an umbra and a penumbra. During a lunar eclipse, the moon briefly passes through the penumbra, where it receives much less sunlight. This causes the moon to appear dim.

Tidal activity, or the periodic rise and fall in sea level, is a result of gravity. It's important to note that tides aren't the same thing as ocean waves. Waves are caused by convection within water and drag from winds, causing water to exhibit a back-and-forth, horizontal motion.

During a new moon phase, the sun's gravity and the moon's gravity combine to produce a spring tide. During the first and third quarters of the moon's revolution around the earth, the sun's gravity and the moon's gravity point along different directions, perpendicular to one another, producing the neap tide. Spring tides produce the greatest difference between high and low tides, while neap tides produce the smallest difference between high and low tides.

The giant impact hypothesis, which currently enjoys favor among scientists, states that a large, Mars-sized object collided with the newly formed earth. (This hypothetical, colliding body is named Theia, after the moon goddess from Greek mythology.) That direct impact completely merged the contents of the two colliding bodies, with debris from the collision coalescing into the current earth and moon. The heat from this collision turned both bodies to a molten state.

fission hypothesis. This idea states that a rapidly spinning, molten Earth spun off a large piece of mass shortly after it formed, and this piece of mass later cooled and formed the moon. George Darwin, son of the famous biologist Charles Darwin, suggested that the earth's moon spun away from the earth early in its formation.

accretion hypothesis, states that the moon and the earth originally formed close to each other from the same material, cooling and solidifying separately. These hypotheses are consistent with evidence that shows rock from the moon and the earth have nearly the same age and composition.

Some have postulated that the moon became captured in the earth's orbit as a result of aerobraking. This is a spaceflight maneuver that uses drag in the atmosphere to decrease the velocity of an orbiting object and change its trajectory.

The moon is tidally locked, meaning that the same side of the moon always faces the earth. Yet the side of the moon facing away from us, often called the "dark" side, isn't perpetually dark. During a new moon, the sun's light illuminates that side of the moon that we never see. The "dark" side of the moon can be explored only with orbiting satellites. Over time, a number of missions have explored and photographed the entirety of the moon's surface.

A telescope is any device that gathers light from a distant object and focuses it into an image. Telescopes can work with visible and invisible light. As you learned earlier, James Maxwell showed in the late nineteenth century that light is an electromagnetic wave, and all light falls into some portion of the electromagnetic spectrum.

As a successor to the Hubble, the James Webb Space Telescope was launched in 2021. This telescope will focus on infrared wavelengths and must be kept extremely cold. To maintain this cold temperature, the telescope will be placed at a specific point in the earth-sun system, called a Lagrange point. Anything placed at this particular point will remain fixed at the same relative location within the earth-sun system. This will allow the James Webb Telescope to continuously shield itself from the sun's radiation so that it can capture more accurate images than Hubble.

These planets are called extrasolar planets, or exoplanets. Locating these potentially life-bearing planets requires measuring a specific event that occurs around distant stars, called a transit. If a planet is orbiting a star, there's a chance that the planet will pass between the telescope and the star, as occurs during an eclipse.

the astronomical unit, or AU. This unit refers to the average distance between the earth and the sun (93 million miles).

Solar systems form from dust clouds that contain elements heavier than hydrogen. Our solar system began forming 4.6 billion years ago following the gravitational collapse of a large cloud of gas and dust. This type of cloud is called a nebula. Many nebulae exist throughout the universe; two examples are the Orion Nebula and the Crab Nebula.

Jupiter, Saturn, Uranus, and Neptune are collectively known as the Jovian planets, and they have different structures than the inner planets. For one thing, the Jovian planets have relatively small, dense cores surrounded by huge layers of gas.

Hydrogen that was located beyond a certain point in the solar nebula, called the frost line, was low enough in temperature to condense and freeze.

The limit to which an object can approach a planet without being destroyed by tidal forces is called the Roche limit, and this limit depends on the density of the object, the density of the planet, and the size of the planet.

The terms asteroid, meteoroid, and comet refer to different objects in the solar system. Asteroids are large rock bodies that never accumulate enough mass to grow into planets. (When an object is massive enough, gravity tends to transform it into a sphere, which is one of the defining characteristics of a planet).

Small rocks and other debris smaller than asteroids are called meteoroids. Most meteoroids that enter Earth's atmosphere vaporize before they ever reach the surface. They create trails of light as they burn, known as meteors or shooting stars.

The most common type of maneuver used to steer probes is called a gravity-assist maneuver. In this maneuver, the probe uses the gravity of a nearby planet to accelerate or decelerate. This alters the trajectory of the probe toward the next point along its journey.

nebula once referred to almost any astronomical object that extended over a large area. Before astronomers knew that galaxies were distant collections of stars, galaxies were called "nebulae" because they appeared fuzzy in telescopes. Today, the word is reserved for clouds of gas and dust.

As the star's hydrogen fuel is depleted and converted to helium, the star begins to cool and expand, forming a red giant.

A more massive star will form a red supergiant. Less massive stars still form red giants, but they're smaller and hotter than supergiants.

The explosion is called a supernova, and it expels heavier elements and radioactive isotopes. The leftover core of the exploded star produces x-rays and gamma rays that can be detected with telescopes.

The mass of the star is already large enough for gravity to compress neutrons to a minuscule volume. Thus, this supernova remnant is called a neutron star. Stars that were originally 10 to 29 times the mass of the sun will end their life cycles as neutron stars.

This forms a planetary nebula in the outer layers. Note that the term planetary nebula has nothing to do with the formation of planets. Rather, these objects were originally mistaken for planets when viewed through small telescopes, and the term has remained. The hot gases in the outer layers move away, leaving behind a compact core called a white dwarf. This core eventually cools and dims.

Stars emit light via two mechanisms: blackbody radiation (also called thermal radiation) and line spectra. Both of these types of light are used to determine the temperature and composition of a star. Line spectra are related to the chemical composition of stars, while blackbody radiation is related to the temperature of a star..

If you use a device called a spectrometer to collect light from the sun, you'll see a broad band that spans throughout the visible region of the electromagnetic spectrum. Some of the light emitted from the sun is infrared, and a small amount is ultraviolet.

The layer surrounding the core is called the radiative zone, so named because energy travels through it as radiation. The radiative zone isn't as dense as the core, although it's still extremely dense. The light, released from the core as gamma rays, travels only a few millimeters in the radiative zone before it's reabsorbed by another atom.

It takes a significant amount of time for light to reach the next layer, called the convection zone, so named because energy is carried through the movement of matter. As photons enter the convection zone—after already having traveled for thousands of years since they were first emitted from the core—the zone heats the gas surrounding it.

The interior portion of the sun's atmosphere, called the photosphere, is responsible for producing the majority of blackbody radiation observed from the sun. The temperature of the photosphere is about 5,800°C.

The outer portion of the sun's atmosphere is called the chromosphere. The chromosphere is an irregular layer above the photosphere where the temperature increases from 6,000 °C to about 20,000 °C.

At these higher temperatures, hydrogen emits light with a reddish color, called H-alpha emission. Emission from this region is seen easily during a total solar eclipse.

The final layer of the Sun is the corona. This layer consists of superheated gases at over 1,000,000 °C and very low density, and this cloud of gases extends millions of kilometers beyond the chromosphere. The corona is very dim and not visible to the naked eye due to the sun's intensity.

The sun is so hot that matter doesn't exist there in any of the three phases—solid, liquid, or gas—that are found on Earth. Instead, matter exists in a state known as plasma. The temperature inside the sun is hot enough that electrons can escape from atoms as ions.

This solar wind originates from ions in the corona. Solar wind primarily originates from the portion of the corona near the poles of the sun, creating regions called coronal holes. Closer to the sun's equator, the magnetic field traps the corona closer to the sun. Coronal holes can be observed from ultraviolet and x-ray images of the sun.

In astronomy, the luminosity of a celestial body is its brightness as observed at a particular wavelength. These two parameters are used to construct a Hertzsprung-Russell diagram, which shows how the size of a star, its luminosity, and its temperature are related.

Brightness is easier to gauge because it's a measurement of the intensity of light passing through a light detector with a small area. The luminosity of an object is the absolute amount of light it emits directly. The intensity of light emitted from a star decreases farther away from the star in the same way that gravity decreases over greater distance. This means that luminosity is more accurate for quantifying the amount of energy released from a star.

the parallax effect. This visual effect is produced as an observer moves, and the position of a distant object appears to change relative to more-distant objects

the theory of relativity, an aspect of which is illustrated by his most famous equation, E = mc2, which is read as "energy equals mass times the speed of light squared."

general theory of relativity, a theory that describes how light interacts with gravity.

This effect is called gravitational microlensing, and it causes light to travel on a curved path rather than along a straight line. It wasn't until 1979 that this effect was observed by examining light passing by massive galaxies. Since that time, it has been used to confirm the presence of binary stars, detect extrasolar planets, and determine the locations of dark matter and black holes.

The Doppler effect is known to occur with sound waves and is used in ultrasonic motion detectors. 

This effect is called a gravitational redshift, in which gravity changes the color of light that's observed from the massive object. The more massive the object, the greater the redshift in light emitted from the object. The relativistic Doppler effect and gravitational redshift can occur together if a massive body is moving at high speed.

The Milky Way is called a barred spiral galaxy. American astronomer Harlow Shapley (1885–1972) made the first reliable measurement of its size in 1917. Our sun is located close to the edge of the Milky Way, and the galaxy's diameter is approximately 100,000 light years. The Milky Way has a thick nucleus in the center, called a galactic bulge, and the galaxy flattens out like a disk the farther out from the center you go.

This center is known as the galactic disk. Celestial bodies in the galactic disk are essentially orbiting the center of the galaxy. There's also a nearly spherical volume of gas and stars that surrounds the Milky Way, called the galactic halo. Two long tails extend from the edge of the spiral pattern and trail out beyond the galaxy, giving an indication of the Milky Way's direction of rotation.

Dark matter can't be observed with light, and it's even more mysterious than dark energy. We mainly know what dark matter is not.

For example, dark matter is not
Visible the way stars and planets are
Made of invisible black holes
Made of dark-particle clouds of normal matter
Antimatter

In addition to the stars discussed in the previous section, the Milky Way and other galaxies contain stars that vary in brightness, called variable stars. Although there are many types of variable stars, all of them can be classified into two types: intrinsic and extrinsic. An intrinsic variable star exhibits varying brightness due to changes in the property of the star itself. In contrast, an extrinsic variable star exhibits varying brightness due to some external factor.

A planet can also orbit a star and move between the star and an observer, forming what's called a transit. The planet momentarily blocks some light from the star as it moves past.

rotating variables are stars that exhibit some variation in their luminosity as they rotate.

Ellipsoidal stars, which have an elliptical outline, also exhibit varying brightness as measured by an observer because they appear to vary in size as they rotate.

Such deformation also affects the temperature of the star, which changes its blackbody spectrum. This type of intrinsic variable star is called a pulsating star.

A particular type of intrinsic variable star is a Cepheid variable. This type of star pulsates radially, meaning that it expands and contracts uniformly along its radius. Cepheid variables are bright and can be seen clearly millions of light years away.

A barred spiral galaxy is only one type of galaxy; other types can be found throughout the universe. In general, there are four types of galaxies: spiral, barred spiral, elliptical, and irregular.

A spiral galaxy and a barred spiral galaxy are very similar. Both exhibit trails of material that trace out the direction of spin. Both can be circular or elliptical, and they spin themselves into a nearly flat disk. These galaxies also have a galactic bulge, a galactic disk, and a galactic halo.

Elliptical galaxies vary greatly in size and mass. The smallest elliptical galaxies are called dwarf elliptical galaxies and may be no larger than globular clusters that contain high amounts of dark matter. Elliptical galaxies are made up primarily of older stars and are largely devoid of gases that would produce stars. They contain leftover gas and dust, which give them a hazy appearance. These galaxies can spin, although the distribution of dark matter within them ensures that they maintain an elliptical shape.

Unlike the other three types of galaxies, irregular galaxies have no regular structure. They also tend to contain large amounts of gas and dust and thus are prime breeding grounds for new stars. In an irregular galaxy, the large amount of dust and gas makes it nearly impossible to distinguish individual stars.

If the universe were static, meaning if it weren't expanding or contracting, one wouldn't expect the superclusters to be moving away from each other in the way that we observe.

Such a model is known as an accelerated expanding universe. It's reflected in the definition of Hubble's constant, where the recessional velocity of an object is related to its distance from Earth.

The second type of decelerating universe is a collapsing universe, in which the universe begins contracting in on itself after an initial period of expansion.