Courtyards at Southpoint
Duke Physicist Describes Vastness of Durham’s Night Sky
The universe is so vast, its size is unknown, Henry Greenside told Courtyards at Southpoint neighbors Feb. 23. But he provided voluminous information scientists have discovered about the scope and nature of our universe.
Greenside, professor emeritus of physics at Duke University, described “A Physicist’s Appreciation of the Durham Night Sky” in a Speaker Series presentation. The Social Committee sponsors Speaker Series events about once a month.
The universe is around 14 billion years old, Greenside said. “If there are any stars more than 14 billion light years away, the light hasn’t reached us yet,” he said. “We don’t know how big the universe is.”
As part of the Speaker Series, Greenside talks about astronomy.
​Citing famed astronomer Carl Sagan, he explained if the age of the universe were compared to the length of a year, the human race did not appear until two minutes before midnight on Dec. 31.
Earth’s sun is but one of billions of stars in the Milky Way galaxy, which is 100,000 light years across. And Milky Way is but one of billions of galaxies in the universe, he said.
“A star is a huge ball of plasma, the fourth state of matter” alongside gas, liquid and solid, he explained. A star produces energy when hydrogen and helium nuclei fuse at the star’s core, and the energy “bubbles to the surface.”
Our sun is 110 times the size of Earth, he reported, noting stars are various sizes. The star nearest Earth is about the size of our planet and is invisible to the naked eye. The largest known star, Betelgeuse, is 900 times the size of our sun.
Even though they burn for billions of years, stars grow old and die, he said. That happens because helium, which is heavier than hydrogen, sinks to the core, pushing hydrogen toward the surface. When hydrogen no longer can get to the core to fuse with helium, a star dies.
“Our sun is a third-generation star,” Greenside said. The original star from which the matter in our sun derives grew up, then exploded and scattered elements across our section of the universe. From that, another star formed and did the same thing. And after that, our sun formed.
The explosions and scattering of elements provide the materials that comprise planets, including the carbon that exists in the solar systems of third-generation stars. “The Earth exists because of third-generation stars,” he said. “All that is extremely well-understood.”
Our sun is a minority among stars because it stands alone, he said, explaining: “Most stars are not by themselves in the sky. Most are pairs, triplets or even sextuplets.”
For example, Algol, the star known as “Medusa’s Eye” in the Perseus constellation, seems to pulsate every three days. Actually, it is composed of “binary twin” stars, which rotate or move back and forth, giving the impression of pulsation.
Our solar system almost had a secondary sun, he reported, noting if Jupiter had been 10 times its size, it would have been a star.
Greenside acknowledged one of the great mysteries is whether life exists elsewhere in the universe.
“There are an immense number of stars in the universe” — 10 to the 22nd power, or 10 sextillion, he said. “Via fusion in its core, each star produces the same kinds of elements with the same abundances.
“Since 1992, we have known most stars have multiple planets” with about 6,000 exoplanets discovered so far, he added. “So, our solar system and Earth do not seem special. Life should exist elsewhere in the universe.”
Life is likely because “life appeared on Earth as soon as life was possible,” he said. Earth was “born” 4.5 billion years ago, and life appeared “shortly after,” about 3.85 billion years ago, when the planet cooled enough to start and sustain life.
Greenside pointed to another question about life in the universe: “Does life have to be like life on Earth?”
“It’s not known,” he conceded. “But let’s first look for Earth-like life using telescopes and our knowledge of biology.”
A couple of difficulties arise, he said. First, Earth-like life might exist but is hard to observe, since it could be (a) underground, (b) deep in an ocean or (c) beneath an ice covering in an ocean. Second, life might not have evolved to be capable of technology that would be easy to detect by Earth humans.
The best places to look for extraterrestrial life are the “green regions” around stars, in spaces where water is liquid, he said.
Turning back to the night sky, Greenside referred neighbors to a website and app, Stellarium, that shows a realistic night sky from the observer’s perspective. It provides multiple resources for seeing the sky and understanding what is seen. To access the website, click here.