A red giant star is an evolutionary stage of a star in which it ran out of fuel and starts burning helium. A red-giant has a mass of about one-half to ten times the mass of our Sun. In this stage, a star will get brighter; sometimes as much as ten thousand times as bright as when it was on the main-sequence. A red giant reaches the sizes of 100 million to 1 billion kilometers in diameter & because the energy is spread across a larger area, surface temperatures are actually cooler, reaching only 2,200 to 3,200 degrees Celsius, a little over half as the sun. This temperature change causes stars to shine in the redder part of the spectrum, leading the name red giant but in fact they are Orangish in appearance.
What if Our Sun became a Red Giant?
Now, what if our Sun became a red giant? Will it engulf everything in its path? Or will it give a new chance to life to exist in outer solar system? Will Earth survive this expansion?
Our Sun was formed 4.6 billion years ago and its total lifespan is around `10 billion years. Since then, Sun has been in what is known as a Main-Sequence star, in which a star burns hydrogen into helium through fusion to create energy & light. Roughly 5 billion years from now, the Sun will enter its red giant phase, in which it will ran out of fuel after burning all its hydrogen into helium. The center will start to get smaller due to sun’s gravity. This will make the layer just outside the center get hotter. This layer will still have hydrogen &it will fuse to make helium. The outside layer of the sun will get much bigger & bigger and will begin to expand.
The sun will grow so large that ii will engulf the closest planet orbiting around it Mercury, then Venus & Earth, maybe even Mars & part or the entire asteroid belt.
Long before Sun will enter its red giant phase; its habitable zone will be gone. Habitable zone or the Goldilocks zone is a region around a star where liquid water can exist on a planet’s surface. Astronomers estimate that this zone will expand past the Earth’s orbit in about a billion years. As star ages, its brightness increases which increases the heat amount a planet receives from its host star. The brightness of our sun is increasing 10% every billion years & this will have deadly consequences for life on Earth. Earth’s Oceans will evaporate & the solar radiation will blast away hydrogen from the water. The Earth will never have oceans again, & it will become a new Venus. When Sun will become a red giant, Earth will become molten & soon it will be engulfed by the Sun. Once inside the sun’s atmosphere, Earth will collide with the particles of gas. Its orbit will decay and it will spiral inward. The engulfing of Earth will shift the new habitable zone that will stretch from 49.4 AU to 71.4 AU- well into the Kuiper belt. During the red giant stage, Earth could expand to an orbit around 50% more distant where it is today. Currently Earth is at 1 AU from Sun & during expands of the Earth’s orbit; it will expand to 1.5 AU. If Earth were just a little further where it is right now to 1.5 AU then we could possibly survive if Sun does not expand to mars or asteroid belt. Imagining in mind that currently Earth is at 1.5 AU & during the expand of its orbit to 2 AU (50% more), then still life will not be possible on Earth.
In 2016, a research was conducted by Ramirez & Lisa Kaltenegger, which was later published in the Astrophysical Journal that titled- “Habitable Zones of Post-Main Sequence Stars”. In this research, the two astronomers looked for new habitable zone after a main-sequence star will become a red giant which may heat frozen worlds into habitable planets.
“Currently objects in these outer regions are frozen in our own solar system like Europa & Enceladus- moon orbiting around Jupiter & Saturn”.
“Long after our own plain yellow yellow sun expands to become a red giant star and turns Earth into a sizzling hot wasteland, there are still regions in our solar system as well as where life might thrive”, said Lisa Kaltenegger.
After Sun will become a red giant, it will warm distant worlds like Jupiter, Saturn, Neptune & their moons in a newly established habitable zone. “For stars like our sun, but older such thawed planets could stay warm up to half a billion years. That’s no small amount of time”, said Ramirez.
The atmospheres of gas giants like Jupiter & Saturn will eventually erode under the increased radiation from the Sun. However, there are gas giant exoplanets called Hot-Jupiter’s which revolve closely around their stars that have managed to hold on their atmospheres. So Jupiter’s fate is far from certain.
When Sun will become a red giant, it will give a second chance for life to evolve in our solar system on one of the 82 moons of Saturn- ‘Titan’. Titan is the only known natural satellite with a significant atmosphere. Conditions on Titan are similar to the Early Earth. During red giant phase of Sun, Titan could enjoy the several hundred million years of potentially habitable conditions with oceans of water ammonia on its surface.
According to Southwest Research Institute astronomer S. Alan Stern when sun will turn into a red giant, the temperature on Pluto, Neptune’s moon & other Kuiper belt objects will be similar to those in tropical locations like Miami Beach on Earth today. Stars spend a few thousand to 1 billion years as red giants, meaning life on the new world could thrive to around a billion years.
After around a billion years, helium in the core of red giants runs out and fusion stops. The star shrinks again until a new helium shell reaches the core. When the helium ignites, the outer layers of star are blown off in huge clouds of gas and dust known as Planetary Nebulae. The core continues to collapse in on itself. Smaller stars such as our Sun will end their life as White Dwarf, while bigger stars end up as Black holes or neutron stars in a Supernova explosion.