Do Stars Move? – Do The Glowing Beauties Jet Across the Sky?
Stars are distant pinpricks of light that have captivated humanity’s imagination for centuries. But they are not as fixed in the skies as they seem. In the vast night sky canvas, we used to think of stars as unchanging beacons. But that notion has given way to a fascinating revelation: stars are in motion.
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The Motion of Stars: Unraveling the Celestial Dance
Let’s delve into the intriguing world of stellar motion. We’ll strip away the mystery to uncover the scientific realities of why and how stars move. We’ll look at why unraveling their movements is important in astronomy. So, let’s figure out why stars move, but first, a quick recap about how a star is born.
Stellar Nurseries
Stars are born through intense gravitational collapse within a dense interstellar gas and dust region. This process begins when particles accumulate, creating a core with immense pressure and heat. As the core temperature rises, hydrogen atoms start fusing into helium. And that releases a colossal amount of energy in the form of heat and light.
This intense energy counteracts the inward pull of gravity and establishes a delicate balance that sustains the star’s stability for millions to billions of years. The nuclear fusion process fuels the star’s brilliance and generates the energy that radiates into space. The star’s mass and composition determine its size, brightness, and eventual fate.
James Webb Stellar Nursery Celebration
The Webb Telescope team released its first-anniversary photograph below to celebrate a successful imaging and data collection year. It shows a star nursery in never-before-seen details. View the cloud complex, Rho Ophiuchi, in this stunning skyscape.
This stellar nursery is the closest one to Earth. It’s a small star-forming region about 390 light-years away. There are about fifty young stars in the image. And most have a similar or smaller mass to the Sun’s. The only more enormous star is S1, and it’s significantly more massive than our Sun. You can see in the image’s lower half where it carved out a dust cave for itself.
The darkest areas in the image are very dense dust cocoons that are still forming protostars. Next, the red jets come from molecular hydrogen that bursts out when stars rip through their cosmic dust wrapping. It’s almost like a newborn baby throwing out their arms. Some stars even exhibit a circumstellar disk where future planetary systems might form.
The Foundation of Stellar Motion
The ancients believed stars were fixed points in the sky, unchanging and eternal. Stars served as guiding markers. They enabled navigation and inspired countless myths and legends across cultures. Their seemingly motionless nature led to the idea of constellations, patterns of stars that appeared to maintain their positions relative to each other.
The prevailing belief in antiquity was that the cosmos itself was immutable (unchanging over time.) Ancient civilizations like the Greeks and Egyptians saw the celestial realm as a perfect and permanent sphere. So, the concept of a static universe reflected the broader philosophical and religious notions of order and stability. And the idea of stars moving or changing challenged this prevailing belief and met with resistance.
The Copernican Heliocentrism Model
The Copernican revolution in the 16th century shattered the notion of an Earth-centric universe. Nicolaus Copernicus proposed a heliocentric model, suggesting that all the planets revolved around the Sun. Copernicus published his model in 1543, On the Revolutions of the Heavenly Spheres. It directly displaced Ptolemy’s geocentric model with Earth as the universe’s center.
Heliocentrism proposed that the Sun remained motionless while the planets orbited in circular paths. Copernicus believed the worlds traveled at uniform speeds, modified by epicycles. And even though he still relied on some of Ptolemy’s incorrect positions, this breakthrough posed a direct challenge to the idea of stars as fixed points.
However, it wasn’t until Galileo Galilei’s telescopic observations that evidence supporting Earth’s motion began to accumulate. Furthermore, Galileo’s discovery of Jupiter’s moons and Venus’ phases demonstrated that celestial bodies could exhibit changing positions and characteristics.
The Copernican model laid the future groundwork for Johannes Kepler’s laws of planetary motion and Isaac Newton’s law of universal gravitation. These breakthroughs provided a comprehensive explanation for both planetary and stellar movement.
As human understanding grew, it became clear that stars were moving and not fixed. But their apparent motion was due to Earth’s own movement and the intricate interplay of gravitational forces within the cosmos.
Do Stars Move? Stars in Action
The phenomenon of proper motion offers a window into the dynamic nature of the ever-moving cosmos. Factors like these influence proper motion:
- Earth’s orbit,
- Stellar parallax,
- And the precision instruments used to measure these celestial shifts.
Do Stars Move? Proper Motion
A star’s proper motion refers to the gradual apparent movement of stars across the celestial sphere over time. It is the star’s angular position changes that occur over a time period. And astronomers measure proper motion in arcseconds per year.
Constellations appear fixed to the naked eye because of their great distances from Earth. But, careful observation reveals that the individual stars gradually shift in position. This phenomenon arises from the combined effects of the stars’ actual motion through space and Earth’s own movement.
Edmund Halley noticed in 1718 that the positions of some stars had changed over the 2000 years since Greek astronomer Hipparchus plotted them. Halley saw the stars’ “proper motions.”
Do Stars Move? Apparent Motion
Several factors influence the apparent motion of stars. Earth’s orbit around the Sun plays a crucial role. As Earth orbits the Sun, nearby stars exhibit a slight annual shift in position due to our changing vantage point.
This phenomenon is similar to viewing nearby objects from a moving vehicle. Objects closer to us seem to move more rapidly than those in the distance. If you’ve ever tried to take a video out your car window, you’ve seen the effects of apparent motion.
Stars appear to move from east to west through the night sky. The movement appears in tracks centered on the North Star. But the star tracks aren’t from the stars moving. Instead, Earth’s axis points to the North Star. So, as the planet rotates, it appears that the stars are moving.
The image below shows this effect more clearly. But rather than the North Star, the ISS poles formed the center of rotation. A series of photos taken by NASA’s ISS astronaut Don Pettit in April 2012 make up this view. As the International Space Station passed over the Caribbean Sea, then across South America, and finally across the South Atlantic Ocean, Pettit took 72 long-exposure images. He later compiled them into this captivating spectacle of star trails.
Measuring Stellar Distances
Stellar parallax is a way of measuring the distances to and between stars. As our planet circles the Sun, a nearby star appears to move against the distant background stars. Say astronomers measure the star’s position now and again in a year. Then they calculate the apparent change in the star’s positioning. Scientists call the apparent motion of the star its stellar parallax. It is the apparent displacement of a star (or object) because the observer’s viewpoint changed.
The greater the observed shift, the closer the star is to us. Here’s another way to see how parallax works.
- Hold your hand in front of you.
- Look at it with one eye closed.
- Then look at it with the other eye closed.
- Your hand appears to move against the background.
The diagram below shows how a nearby star looks as if it moves against the distant stars when Earth is at different orbital positions. Like your hand, the star isn’t moving. But your perspective changes, so you see apparent movement.
The Role of Precision Instruments in Measuring Proper Motion
The accurate measurement of proper motion requires sophisticated and precise instruments. Modern telescopes equipped with digital sensors and specialized cameras let astronomers capture high-resolution images of star fields. The Hubble view of the cloud complex Rho Ophiuchi above is a testament to the complexity of precision instruments.
Scientists compare current images with historical records. Often the comparisons span decades or even centuries in order to detect the tiny shifts in star positions.
Gaia
One notable instrument today is the astrometric satellite, Gaia. The European Space Agency launched the spacecraft in 2013 on a global learning mission. It is creating a three-dimensional Galaxy map and surveying a couple of billion space objects to do so.
Gaia’s unprecedented precision provides an enormous dataset of stellar positions and motions. It is revolutionizing our understanding of the Milky Way’s structure and the dynamics of its stars. The satellite can measure stellar positions with microarcsecond accuracy. And that has led to the discovery of thousands of previously unknown binary stars and exoplanets.
One of the many cool features of Gaia is the Radial Velocity Spectrometer (RVS) tool. It enhances scientists’ ability to measure radial velocity, the motion of stars toward or away from us. Combined with proper motion data, we get a more complete understanding of how stars move in three-dimensional space.
Gaia monitors targeted stars about 14 times each per year. It charts their movements, positions, and distances. The satellite also notes any detected brightness changes, which could lead to the discovery of an orbiting exoplanet. The research team expects Gaia to continue discovering new space objects. In 2014, it discovered a supernova in a distant galaxy. And in 2015, the team released a star density map of the Milky Way.
A big surprise to researchers is that Gaia can detect tiny motions on a star’s surface. These starquakes change the star’s shape. However, the Gaia observatory wasn’t initially built for this type of discovery. So the results are amazing.
Everything we observe throughout space is moving. The stars move, the Sun moves, and so does our solar system. Gaia mainly observes movements around the Milky Way’s core. But those movements have considerable variations. The movements come from conditions during the star’s formation and any encounters it experienced during its lifetime to date.
Scientists measure space motions in a couple of ways. First, they measure an object’s radial velocity along the line of sight. They examine the Doppler shift in the object’s (star’s) spectral lines. When an object moves toward Earth, the spectrum shift is shorter, a blue wavelength. However, if the star is moving away from Earth, its absorption lines shift toward long, red wavelengths.
Next, space velocity appears as a displacement in the sky, like the movement of your hand when closing one eye. That’s the star’s proper motion. And it scales with distance, so it will be smaller when the object is seen at greater distances. On the other hand, the shift appears larger when the object is closer to the observer.
Galactic Motion: Stars in the Milky Way
Our home galaxy, the Milky Way, is a typical spiral system. It contains a central bulge, a nucleus, a disk, two main spiral arms with multiple minor spurs, and a giant halo. Here are three views of our galaxy from differing perspectives: face-on, edge-on, and the apparent shape. It helps explain how we can “see” the Milky Way from Earth even though we reside within it.
The Sun orbits the Milky Way’s center, and it takes everything within our solar system along with it. So in basic terms, the Moon orbits the Earth. Then the Earth orbits the Sun. And the Sun orbits the galactic center.
Our whole solar neighborhood travels through space together. Let’s look at some speeds.
| Celestial Object | Axis Rotation Speed | Orbital Rotation |
|---|---|---|
| Mercury | 6.77 miles (10.892 km) per hour | 105,946 mph around the Sun |
| Earth | 1,000 miles (1,600 km) per hour | 66,622 mph (107,000 km/h) around the Sun |
| Neptune | 6,039 miles (9,719 km) per hour | 12,158 mph around the Sun |
| Sun | 1.241 miles per second | 137 miles (220 kilometers) per second around the galactic center |
Understanding how our solar system objects rotate and travel through space gives a better idea of how much stars move. Scientists use our solar system as a baseline for learning about stars and worlds beyond our universe.
Stellar Populations Beyond Our Solar System
Astronomers call the space between galactic stars interstellar space. The term itself instills a sense of awe and wonder about what else is out there.
Proxima Centauri
Between the Sun and its closest neighbor lies about 4.2465 light-years of interstellar space. Based on Gaia’s Data Release 3, Proxima Centauri is a red dwarf in the southern Centaurus constellation.
Proxima Centauri moves across the sky at 3.85 arcseconds per year with a fairly large proper motion. The plot below comes from 2013 Hubble Space Telescope observations of the star. The green line predicted how the star would move over the next ten years.
The path appears scalloped, but that’s due to the parallax you see from Earth’s movement around the Sun. And because this star is the closest one to the Sun, its angular motion through the sky appears fast compared to the distant stars in the background.
One benefit of projections like this one is that scientists used it to plan when Proxima Centauri would pass before two background stars along its path. Scientists used the knowledge to study how Proxima’s gravity warped the space around it. The warping shows an apparent displacement of the stars in images. (Think about holding up your hand and closing one eye.)
As it turns out, in 2013, astronomers saw a wobble that could mean a nearby planet to the red dwarf. And then, in 2016, confirmation of an exoplanet arrived! Here’s an artist’s illustration from the European Southern Observatory of how it might possibly look.
Barnard’s Star
Astronomers found Barnard’s Star in the Ophiuchus constellation, about 5.96 light-years away. It’s the fourth nearest star to the Sun beyond Proxima Centauri and its two neighbors within the Alpha Centauri system.
Barnard’s Star moves at a whooping fast speed of 10.3 arcseconds per year. That means it would take only about 180 years to cross the Moon’s face. Other stars have much smaller proper motions and would cross over longer periods.
Astronomers use Gaia’s measured radial velocities and distances with the proper motion to determine a star’s true movement through space over the years. The data helps provide details of how our own Sun moves through space. And it also helps identify distant unusual stars that move differently from our closer neighbors.
Barnard’s Star gained the nickname of the Great White Whale of planet hunting. Scientists looked for exoplanets near the star for years before finding a super-Earth-sized planet. It lies just beyond the star’s habitable zone and orbits Barnard’s every 233 Earth days.
Large Magellanic Cloud
Scientists used Hubble data to measure a galaxy’s rotation based on how its stars move. Analysis showed that the central part of the Large Magellanic Cloud (LMC) completes a full rotation every 250 million years. That’s about the same amount of time it takes the Sun to rotate around the Milky Way’s center.
The Hubble team used the telescope to measure the average movement of hundreds of stars within the LMC. It recorded slight motions in the stars over a seven-year timeframe.
Researchers say that studying the nearby (170,000 light-years away) galaxy through tracking how the individual stars move provides insight into the disk galaxy’s internal structure. The galaxy rotation rate shows how the galaxy formed. And scientists even use the rotation rate to calculate the galaxy’s mass.
Stellar Collisions and Close Encounters
You may wonder if stars ever collide since they move through space. And the answer is yes, stars move and occasionally collide. It is rare, but when it happens, neutron star collisions release an immense amount of heavy materials. They become the new building blocks of space.
In August 2017, ground- and space-based telescopes and observatories captured the merging of two neutron stars about 130 million light-years from Earth. They were the shrunken leftover cores of two massive stars. First, they orbited one another hundreds of times per second. It was a close swirling dance, causing gravitational waves (or ripples) through space.
Finally, the stars spun quickly enough and close enough to break apart and merge. The resulting energy flew into space as a gamma-ray burst and a bright flash of light, a kilonova.
Many Earth and space detectors and telescopes captured the explosion and detected the gravitational waves. The comprehensive observations from many avenues help scientists test conclusions about what happens when neutron stars collide and merge.
Do Entire Galaxies Collide?
Not only do stars move through the skies, but the galaxies that contain them also move. The Milky Way spins at about 130 miles (210 km) per second. But some super spirals spin even faster, up to 350 miles (570 km) per second (570 km.) So what happens when they come near each other?
When two galaxies collide, there isn’t as much damage as you might first think. For example, astronomers predict a future collision between the Milky Way and Andromeda galaxies. Now, before you start making a doomsday plan, this isn’t happening for almost four billion years.
The Sun might get flung into a new area of the galaxy, but scientists think the solar system has no imminent danger because of the collision. The Andromeda galaxy lies about 2.5 million light-years from Earth. But the galaxy is falling toward the Milky Way because of the mutual gravity pull of both galaxies and their surrounding dark matter.
Because stars are so far apart from one another, scientists think it is unlikely that they will collide during the galaxy merger. But our night skies would definitely look different from how they are today! Check out this potential version.
Do Stars Move?: Stars in an Expanding Universe
The expanding universe theory proposes that the fabric of space itself is stretching, causing galaxies to move away from each other. The Hubble Telescope gave surprising calculations that the universe is expanding faster (instead of more slowly) than scientists thought.
One thing is sure about space exploration, and that is continual technological advancements change the status quo. Hubble’s Law, formulated by astronomer Edwin Hubble in the early 20th century, solidified the expanding universe theory.
He observed that light from distant galaxies appeared redder than expected, a redshift. It indicated that galaxies were moving away from us. This redshift comes from the stretching of space that causes light waves to lengthen as they traverse the expanding universe. Hubble’s Law quantifies this relationship between a galaxy’s distance and its redshift.
Albert Einstein also realized that black space isn’t “nothing.” Instead, it is full of dark energy and dark matter. He discovered that more space could come into existence. But Einstein also predicted a cosmological constant – space that appears empty can possess its own energy.
Furthermore, the energy wouldn’t dilute as space expands because it is a property of space itself. So, as more space exists, more energy appears, meaning the universe should grow faster. But no one fully understands what the cosmological constant is.
Within the grand scheme of cosmic expansion, the Local Group—an assembly of galaxies including the Milky Way and Andromeda—experiences its own motion. While galaxies within the Local Group are gravitationally bound, the universe’s expansion exerts an additional influence. Despite their gravitational pull, galaxies beyond the Local Group move away, causing a relative motion.
Over billions of years, the Local Group will continue interacting with neighboring galaxy clusters and experience the overarching flow of cosmic expansion.
Conclusion: Do Stars Move?
Stars are not static fixtures but moving participants in an ever-changing cosmic tango. This realization is a far cry from ancient understandings of space. Nicolaus Copernicus proposed a new heliocentric model of the skies in 1543, revolutionizing astronomy. Understanding how our solar system moved led future astronomers to extrapolate the information to stellar motion.
Proper motion allows scientists to measure observed changes in a star’s apparent placement in the sky. It reminds us that just because we see stars in one place, it is actually relevant to Earth’s position.
We can expect more in-depth discoveries as earth and space-based observatories continue gaining technology. How stars move teaches us about their formation and even helps predict their future life cycles. It’s a fascinating time of discovery!