Sun was too hot a million years ago!?

I know scientists date the Earth at millions upon millions of years old. Question is: Based on current measurements of the sun, wasn’t the sun too big and hot back then to support any living thing on Earth?

I believe that the Earth is thought to be 3 Billion years old, and I don’t think the Sun has changed much since the earth was formed so no, life could have formed a Billion years ago… easily.

I think this will turn out to be a case of extrapolating too small a data set in the assumption that a very few observations are indicative of a uniform trend.

For example; On Thursday, the milkman left one bottle of milk on my doorstep, the next day he left two and on Saturday, there were four bottles; based on this pattern, I calculated that on the last day of March, he will be delivering 8,589,934,592 pints of milk to my house. Naturally, I was concerned about this and contacted the dairy with all haste, but they laughed and told me that they have been delivering one pint per day on Monday to Thursday, two on Friday, four on Saturday and none on Sunday, and will continue to do so until instructed to the contrary.

Current measurements say the Sun is getting hotter, not cooler. In any case, life has been extant on Earth for about 3.5 billion years, so I’d say, no, the Sun has never been too hot for life in the past million. The Earth is believed to be about 4.5 billion years old.

Are you suggesting that the sun is gradually getting smaller and cooler?

There was plenty of life on earth one million years ago and earlier. Remember the dinosaurs are 65 million+ years old. The earliest life is thought to have spung up 3 billion or so years ago. Almost since the planet cooled down it has been supporting life, so there is no evidence that the sun was ever too big or too hot for life on earth.

As stars get older the energy they emit increases. This counter acts the gravitational attraction of the star’s mass and so moves the surface of the star outwards. Since the surface areas has increased the surface temperature drops. But the total energy emitted has gone up.

I believe that the sun emitted 70% of its current energy 3-4 billion years ago. However since the original atmosphere of the air is thought to be primarily methane and carbon dioxide a strong greenhouse effect would’ve been in place to maintain viable temperatures.

I agree with your timeline, but the general stellar spectrum (OBAFGKMS, Q) would suggest the sun is cooling down - regarding measured surface temp. Granted, Grey’s post probably clarifies the matter because temp is not a direct measure of energy, but a measurable result. (There’s no “energy-momter” we can stick into any system to measure energy directly.)

Regardless, the question remains: What was the sun like 3.5 billion years ago?
I should mention that, having read-up on this recently, the biologists’ timeline show that “life” is believed to be only single-celled organisms surviving by chemosynthesis - living off sulfur from steam vents at the bottom of the seas. These were the eucaryote cells, IIRC? Next, autotropic cells (capable of photosynthesis) came next. They were the procaryotes, IIRC. (I can cite a ref text later.)

Even going back to 4.5 billion years ago, it’d be interesting to know more about the dynamics of a slowly-cooling earth, steam vapor atmosphere (no oceans yet) and related greenhouse effect (at least, to some degree), and a younger sun. We can only assume, from empirical evidence, the sun’s influence was not a precluding factor, but maybe a hindering factor? Maybe events would have played out faster on the biologists’ timeline, under a cooler sun?

Very interesting OP!

  • Jinx

See The Incredible Shrinking Sun at for an answer to this question.

I don’t understand how the stellar spectral classes could suggest that the sun is cooling down. Makes not the slightest bit of sense to me.

BTW, the order of spectral classes is OBAFGKMLT where L and T are new classes for brown dwarfs.

No, you got the names backwards. Procaryotes were first, starting about 3.5 billion years ago. Eucaryotes came after, perhaps about 1.5 billion years ago.

Procaryotes include two groups: the Archaea (formerly included among the bacteria), which are today restricted to extreme environments (hot, hypersaline, etc) and which some biologists think may be the oldest form of life, perhaps originating near hydrothermic vents on the sea floor; and the Bacteria, including the cyanobacteria, or blue-green “algae.” The cyanobacteria are thought to have been the first photosynthetic organisms.

Eucaryotes include all other life on Earth, including one-celled forms such as “protozoa” and algae as well as the multicellular plants, animals, and fungi. Eucaryotic cells are thought to have originated through sequential symbioses between several different kinds of bacteria.

Jinx: The earliest life forms were prokaryotes, a fancy way of saying bacteria. Eukaryotes are more complicated organisms that have nucleated cells. Animals, plants, protozoans and Fungi are Eukaryotes, bacteria are prokaryotes.

The current estimate for the age of the Earth is about 4.56 billion years. The Sun is presumably a bit older than that.

There are a number of models for the evolution of stars, but for a G-type star like the Sun, the estimated luminosity (brightness) at the time the Earth was formed ranges between 25 and 30 percent less than the current value. The discordance between a considerably fainter Sun, and the inferred presence of liquid water on the surface of the Earth within a few hundred million years of planetary formation, is referred to as the “faint young Sun” paradox. Most models that attempt to reconcile the paradox involve an Earthly atmospheric composition that is high in a strong greenhouse gas like methane or carbon dioxide. Given that there are so few rocks remaining from that part of Earth’s history (and these too highly metamorphosed, or altered), it’s unlikely we’ll ever have a definitive answer regarding early Earth’s environmental conditions.

On preview, I see that Colibri and Lemur866 have clarified the prokaryote/eukaryote issue. BTW, the dates of the earliest known fossils are subject to some dispute where the evidence put forth is based on chemical signatures in rock alone (these reportedly go back as far as 3.5 billion years). The oldest known actual microfossils, where you can see cellular structure-like things under a microscope, date back to about 2.5 billion years, IIRC. Early organisms would have been chemotrophs (if not chemosynthesizers). Some researchers believe that single-celled photosynthesizers may have existed as early as 2.8 billion years ago, but that their numbers didn’t reach a significant level until about 2.2 billion years ago, when atmospheric oxygen levels seem to have increased significantly from negligible amounts to perhaps as much as 10% of present atmospheric levels.

Getting back to conditions for early life on Earth - a key question is, how harsh is too harsh? We have organisms today - extremophiles - that are capable of thriving in temperatures near the boiling point of water, in glacial ice, deep underground at tremendous pressures, and in waters so acidic the pH is less than 1, as well as capable of surviving gamma radiation at many thousands of times the levels tolerable by humans. Molecular biologists are trying to determine, through examination of RNA/DNA and evolutionary relationships, what sort of critter would have been among the earliest to survive here, but so far have not discovered any strong clues about the evolutionary environment. The assumption that life arose on Earth near undersea thermal vents is based on the thought that deep sea organisms might best be able to survive continuing debris impacts on the Earth’s surface, but there are currently no data that strongly support this idea to the exclusion of other possibilities.

On the other hand, you can’t have life as we know it developing without liquid water. So, even with a cooler Sun, the surface temperature conditions on early Earth couldn’t have been too much different (i.e. cooler) than today, through the compensating effect of greenhouse gases. It doesn’t appear to me the fainter Sun should have been a significant evolutionary pressure in and of itself as long as greenhouse compensation was a factor as well.

When I learned this it was OBAFGKMRNS. Nice to know they’ve added L and T, but what happened to RNS?

Comment acknowledged, but I have to get to my refs this evening before responding… - Jinx

There’s a not-uncommon misconception out there that a star goes through the spectral classes as it ages; I know this because I held this misconception until I actually took an astronomy class in college, when I was gently disabused of the notion. From what I remember the prof telling me, this idea was put forth in the sixties, but was discredited sometime after that. Maybe someone will be along soon to elucidate my memories…

That is correct. It is highly unlikely that our sun was ever, at any time, an O or B class star.

RNS are still there, but the usual way to write them is to put them in parens: OBAFGKMLT (RNS). This shows that they different from the rest of the sequence which goes from hottest to coolest.

The main characteristics of a star’s spectrum primarily reflect the surface temperature of the star. For instance, stars at a certain temp display very strong hydrogen lines and these are A stars (originally the letters were assigned based purely on the strength of the hydrogen lines). After rearranging the letters to reflect the temperature, they ended up with the order OBAFGKM.

But there are some stars that have spectral features that do not depend on just temp, and that’s what the R, N and S classes are. They are basically highly evolved stars on what is called the Asymptotic Giant Branch (temperaturewise, they belong in the K and M classes). Here’s a page describing spectral classes including RNS: The Classification of Stellar Spectra

Simple explanation: Spectral classification is a system for classifying stars according to surface temperature (Ref: "Complete Idiot’s Guide to Astronomy (c) 2001) The Hertzsprung-Russell (H-R) Diagram is based on this with “O” are the hottest stars at about 35000 K and “M” stars are coolest at about 3000 K. It is well known that the sun is an average star lying in the “main sequence” on the H-R diagram. It was hotter when younger; it will be cooler as it ages. There is agreement on this across many references. (The sun is “G” class about 6000 K at the surface.)

As L and T, I could not find info on this at this time. However, it’s hard to imagine how a brown dwarf can have a spectral class when it is defined as “a failed star where forces of energy and gravity rose to equilibrium before the core temp could rise sufficiently to trigger nuclear fusion in the core.” (Ref: ibid.)

An older book mentions odd classes, such as “Q”, “W”, and “S” for rare stars. The mnemonic used to be “Oh, Be A Fine Girl Kiss Me! Smack!”. I will try to confirm if these lesser-known spectral classes are still used. And, I have a friend studying the sun for NASA who may have a moment to address this question by email - and I can relay the response. - Jinx

Just saw the above comment. This is true, and I meant to include a mention of R and N in my previous post above… - Jinx

I knew I’d get the two terms mixed up! Thanks for the corrections, Lemur866 and others who pointed this out. But, these terms refer to single-celled organisms. Animals are not eukaryotes, for one. I believe I can provide a more-precise, clarifying definition from a ref I can cite - pending (ASAP). - Jinx