Bubbles, jets and stellar infancy


The Hubble Space Telescope (HST) has been exploring the universe for over three decades. Its recent troubles suggest that it is showing its age. Every time a team of scientists announces a startling discovery garnered from Hubble data, I can’t help but think, “Will this be the last?”

It’s tough to pick the HST’s most significant contribution to astronomy, but I’ll try. My mind always wanders back to the images and movies of two newly born star systems, XZ Tauri and HH 30, throbbing in their violent infancy.

A star like our sun is an extraordinarily violent object — a bubbling, boiling, heaving cauldron of thermonuclear energy. Even the faintest of stars is essentially a hydrogen bomb exploding with the power of many trillions of our earthly H-bombs every second.

As violently explosive as the sun is, its current state is practically somnambulant compared to its first few million years of life. Stellar babyhood is highly chaotic.

To understand a star’s infancy, we must begin even before it is born.

These days, stars start their lives in the enormous gaps between the already-existing stars. Astronomers have come to call that space the interstellar medium.

In that regard, “empty” space is not entirely empty. Photons, subatomic particles of radiated light generated by stars, zip through it at the speed of light. Cosmic rays, atomic nuclei generated by exploded stars many light-years away, stream through space. Particles of dust and gas are present in even the “emptiest” of volumes.

Some parts of the interstellar medium are a bit more crowded than the rest. Stars form in voluminous, amorphous collections of mostly hydrogen gas called molecular clouds, which some astronomers call stellar nurseries.

The clouds also contain trace amounts of dust, which form the fundament of planetary formation later on.

Massive, supergiant stars live and die within a few hundred million years. During their relatively short lifespan, they create the heavier elements that make up planets. When they die, they explode, thus seeding molecular clouds with dust.

Molecular clouds range in size from 20 to 600 light-years in diameter. (One light-year equals about six trillion miles.) By comparison, the distance from the sun to the nearest star, Proxima Centauri, is 4.2 light-years.

Even tiny molecular clouds contain, in total, more than 10,000 times more material than our sun. However, those molecules are spread out over the entire volume of the cloud.

Still, a molecular cloud is far denser than the interstellar medium. On average, a cubic centimeter (.06 cubic inches) of the interstellar medium within our solar system contains one molecule. The same space in a molecular cloud encompasses hundreds or even thousands of molecules.

The numbers might convince you that molecular clouds are dense. Think again. A drop of water on Earth contains 1.5 sextillion water molecules. That’s dense.

Significantly, a molecular cloud contains clumps of gas and dust a little thicker than the rest of the cloud. Astronomers have named them, not very inventively, clumps. If conditions are right, stars — with their attendant planets — form from those clumps.

One of those conditions is temperature, which is an indication of how fast the molecules are moving. Higher temperatures mean faster molecular motion.

If the molecules in a clump are moving too fast, they will overcome the clump’s tenuous gravity. The clump will not collapse into a star.

Molecular clouds are frigid by any standard — about 18 degrees above absolute zero on the Fahrenheit scale, about -442 degrees Fahrenheit.

As a result, a clump’s weak, collective gravity is powerful enough to collapse the clump into a swirling disk.

At the center of the disk, most of the material collapses into a spinning sphere of even denser gases. Deep in the sphere’s core, the hydrogen gas gets so hot that hydrogen fuses into helium.

The resulting release of energy fuels the star’s explosive power. The sheer quantity of hydrogen collected ensures that its thermonuclear reaction can last billions of years.

As the star develops toward middle age, some leftover material in the outer disk condenses into spheres of non-exploding, cooler material. Because the disk was swirling in the first place, the balls take up mostly stable orbits in the direction of the initial swirl. We have come to call those orbiting balls planets.

Stars like our sun have settled down to a respectable and relatively sedate middle age. At about five billion years old, the sun has achieved a miraculous and delicate balance between the explosive power that wants to tear it apart and the enormous gravity that wants it to implode to a tiny, dense sphere.

It was not always so. In a star’s youth, it is even more violent than in its middle age. As images and movies from the Hubble Space Telescope illustrate, infant stars and their surroundings change spectacularly in just a matter of weeks. The surfaces of those stars erupt with gigantic jets and bubbles of scorching gas, which plow into the surrounding space at hundreds of thousands of miles per hour.

Those enormous eruptions may be a natural circumstance of a star’s birth trauma.

XZ Tauri and HH 30, two young star systems featured in some of the Hubble Space Telescope’s most spectacular images and movies, are prime examples of T Tauri stars, about which I have written previously.

Suffice it to say that T Tauri stars are very young, less than 10 million years old. They fall in the range between .5 and three solar masses. The sun fits into that range at one solar mass, so our day star may have once gone through the T Tauri stage.

Early in their development, T Tauri stars have not yet developed core temperatures sufficient to create the thermonuclear reaction that would make them mature stars. However, they have developed enough gravitational oomph to generate a considerable amount of energy, as we shall see.

XZ Tauri and HH 30 are about 450 light-years from Earth. They reside in the Taurus-Auriga molecular cloud, one of the nearest stellar nurseries to our planet.

XZ Tauri is a binary star. We get two T Tauri stars for the price of one. The two stars orbit each other, kind of like your thumbs twiddling, at a distance of 3.6 billion miles, about the span between dwarf-planet Pluto and Earth’s sun.

HH 30 is a single star.

All three stars are probably less than a million years old. Stars of their mass typically last for billions of years. That makes them relative newborns.

When stars in their size range are still young, vast quantities of residual gas still surround them. The stars have not yet formed planets.

Gas and dust are still flowing toward the stars from their adjacent disks. But not all the gas makes it there.

Inside the stars, gravity compresses the hydrogen. As a result, temperature and density increase dramatically, so much so that the stars’ constituent hydrogen can conduct electricity, a state called metallic hydrogen.

As the stars rotate and hydrogen churns around their outer layers, the stars generate enormous quantities of electricity. Their powerful electric currents generate massive magnetic fields.

The stars’ magnetic fields grab some of the gas from the stars’ outer envelopes. That doomed material passes along the lines of magnetic force surrounding the stars and is then blown outward into space at their north and south magnetic poles.

The Hubble images reveal the evolution of the jets of glowing gas heaving outward from young stars. A typical outburst may last only weeks, but the burst pulses repeatedly with the ebb and flow of the star’s constantly magnetic field and the build-up of gases near the star.

The time-lapse movies of XZ Tauri reveal a heaving bubble of hot gas being forced 60 billion miles into space at almost 300,000 miles per hour.

The images of HH 30 suggest that these outflows are sometimes (perhaps even often) magnetically squeezed into narrow jets that can extend trillions of miles away from the star.

If you have ever wanted to appreciate the power of a star, the HST images are worth a look. You webheads out there can find an image of XZ Tauri at https://esahubble.org/videos/heic1424a.

For a stunning animated version of a gas bubble blowing out from XZ Tauri, go to https://spaceflightnow.com/news/n0009/21hstmovies/movie1.html.

For a look at HH 30’s polar jets and the disk of pre-planetary material, go to https://esahubble.org/images/opo9524e/.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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