Stars and their formation remain key to understanding everything we know about solar systems like ours. But how stars form and ultimately spawn rocky planets on which life can flourish is still not understood with the kind of certainty that one would expect in this age of high-powered computer simulations and space-based telescopes.
Stars actually get their start in giant molecular clouds of gas and dust, like those found in the nearby Orion Nebula. Roiling forces deep inside these giant clouds cause matter to knot into clumps that eventually collapse under the weight of their own gravity. Not unlike empty buildings undergoing controlled demolitions, as these clumps collapse, their centers begin to heat and differentiate into hot cores and protostars.
Then over timescales of tens of millions of years, these protostars turn into full-blown hydrogen-burners, fusing hydrogen into helium. The remaining gas and dust around these protostars can in theory form planets, asteroids or even comets.
Yet many questions remain, including the following:
— Why is the cosmos so predisposed to making low mass M dwarfs? That is, cool red dwarf stars which range in mass from as small as a tenth up to about half the mass of our Sun.
“This influences almost everything in astrophysics, not just the number of potentially habitable planets, but the distribution of heavy elements over space and time in the universe, the supernova rate, and the properties and evolution of galaxies,” Stella Offner, an astronomer at the University of Texas at Austin, told me.
These red dwarf stars make up an estimated 75 percent of stars in the cosmos. But why? One idea which I noted in a recent issue of Astronomy magazine is that the most common stellar mass is somehow correlated with the minimum mass at which nuclear fusion can begin. However, there is no theoretical agreement on this conundrum.
— Why is the formation of stars so inefficient?
The giant clouds that form stars can contain tens of thousands of solar masses of gas, says Offner. But she says observations suggest that less than 5% of this gas actually forms stars.
That’s partly because these giant star-forming molecular clouds are transient, Sally Dodson-Robinson, an astronomer at the University of Delaware in Newark, told me. These clouds, she says, get dispersed by galactic tides, blown away by winds from the young, intermediate-mass stars they form, and most dramatically get broken up by supernovae.
And as Dodson-Robinson points out, when the universe had nothing except hydrogen, helium, and trace amounts of lithium, there was no easy way to cool down clumps of dense gas enough for gravity to take over and make them collapse. That’s because heavy metals act to absorb a cloud’s heat, enabling it to cool until it becomes gravitationally unstable.
— How does a star’s metallicity content impact the production of habitable planets?
“Planet-forming disks with high concentrations of elements such as silicon, iron, and oxygen have more raw material for planet formation than disks with low metallicity,” said Dodson-Robinson.
But because the galaxy’s metal content built up as byproducts of supernovae over cosmic time, Dodson-Robinson says that planets that formed early in the galaxy’s history may be silicate-rich and lack the metallic iron-nickel core that is a signature characteristic of our own Earth. She notes that it’s Earth’s liquid outer iron-nickel core that enables our protective geomagnetic field which deflects charged lethal solar wind particles and cosmic rays.
So the universe’s very oldest planets, says Dodson-Robinson, might not be as protected from harmful particles as we are.
— Do we have a better understanding of planet formation than star formation?
Katelyn Allers, an astronomer at Bucknell University in Lewisburg, Pa., says no. We know much more about star formation than we do about planet formation, Allers told me. For one thing, astronomers can study stars as they are forming. For planets, she says, we only have a few examples that might indicate exoplanets in formation. So, she says, we don’t have as much empirical data on planet formation.
“Given the great differences between the planets and moons in our own solar system, it’s difficult to develop a theoretical model that explains all types of planets,” said Dodson-Robinson.
That’s not to say that stars are simpler than planets, she says, but the mass of a star is incredibly deterministic. In essence, Dodson-Robinson says that a star’s mass controls its luminosity and lifetime.
Yet Offner says we are still missing quite a bit when it comes to understanding star formation.
Even with the fastest supercomputers, Offner says we are not able to follow the scale of an average stellar birth cloud, of some 30 light years in diameter, down to the formation of an individual star. And because gas and dust obscure the earliest stages of star formation on scales of less than a 100 Earth-Sun distances, astronomers also miss such observational details, Offner says. This is a problem for both stellar theory and observations, she says.
Ultimately answering such questions may help us understand what sort of life might start where and around what type of star. We do know that life here on Earth was profoundly affected by our Sun’s peak radiation wavelengths.
“Light with wavelengths between 400 and 700 nanometers travels through earth’s atmosphere very well, so our eyes use that kind of light to see,” said Dodson-Robinson. In contrast, life on a planet orbiting an M dwarf star, she says, would have no need to develop vision in the optical or ultraviolet because red dwarfs don’t put out much optical light.
By the same token, Dodson-Robinson says that although we haven’t needed to evolve radiation-protective skins, that doesn’t mean it can’t be done.
Many astrobiologists think that habitable zones around orange dwarf stars are still best for the evolution of intelligent life. That’s because, among other things, they are a bit longer lived than our Sun.
“Given that intelligent life has taken pretty much half of our Sun’s lifetime to develop, having a longer lifetime is certainly a plus,” said Allers.
But as Offner points out, we’re here around a G-type star talking about all this. Thus, she argues, stars longer lived than ours aren’t a prerequisite for the evolution of intelligent life.
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Stars and their development stay essential to comprehending whatever we understand about planetary systems like ours. However how stars form and eventually generate rocky worlds on which life can grow is still not comprehended with the sort of certainty that a person would anticipate in this age of high-powered computer system simulations and space-based telescopes.
Stars in fact get
their start in huge molecular clouds of gas and dust, like those discovered in the neighboring Orion Nebula. Roiling forces deep inside these huge clouds trigger matter to knot into clumps that ultimately collapse under the weight of their own gravity. Not unlike empty structures going through regulated demolitions, as these clumps collapse, their centers start to heat and distinguish into hot cores and protostars.
Then over timescales of 10s of countless years, these protostars become full-blown hydrogen-burners, merging hydrogen into helium. The staying gas and dust around these protostars can in theory type worlds, asteroids or perhaps comets.
(************ )Yet numerous concerns stay, consisting of the following:
— Why is the universes so inclined to making low mass M overshadows? (*************** ) That is, cool red dwarf stars which vary in mass from as little as a tenth as much as about half the mass of our Sun.
“This affects practically whatever in astrophysics, not simply the variety of possibly habitable
worlds, however the circulation of heavy aspects over area and time in deep space, the supernova rate, and the residential or commercial properties and development of galaxies,” Stella Offner, an astronomer at the University of Texas at Austin, informed me.
These red dwarf stars comprise an approximated 75 percent of stars in the universes. However why? One concept which I kept in mind in a current concern of Astronomy publication is that the most typical outstanding mass is in some way associated with the minimum mass at which nuclear blend can start. Nevertheless, there is no theoretical arrangement on this dilemma.
— Why is the development of stars so ineffective?
(************ )The huge clouds that form stars can include 10s of countless
solar masses of gas, states Offner. However she states observations recommend that less than 5% of this gas in fact forms stars (************************ ).
That’s partially due to the fact that these huge star-forming molecular clouds are short-term, Sally Dodson-Robinson, an astronomer at the University of Delaware in Newark, informed me. These clouds, she states, get distributed by stellar tides, blown away by winds from the young, intermediate-mass stars they form, and the majority of drastically get separated by supernovae.(********* )
And as Dodson-Robinson explains, when deep space had absolutely nothing other than hydrogen, helium, and trace quantities of lithium, there was no simple method to cool off clumps of thick gas enough for gravity to take control of and make them collapse. That’s due to the fact that heavy metals act to soak up a cloud’s heat, allowing it to cool till it ends up being gravitationally unsteady.