• Just like you and me, stars change over time.
• By studying the characteristics of stars, like their temperature and luminosity, astrophysicists figured out how stars evolve over time.
• This amazing insight is the primary lesson of the Hertzsprung-Russell (HR) diagram.

Human beings, as the species Homo sapiens, have been around for about 300,000 years. That turns out to be about 100 million nights during which somebody, somewhere looked up at the dark sky and asked, "What are those twinkly lights?"

Given all those nights and all those people asking pretty much the same question, it is pretty remarkable that we happen to live in one of the first generations that actually knows the answer. Here in the 21^st century, we know for sure what stars are, and a key reason we have that knowledge is because of a little something called the HR diagram. Over the summer, I wrote two other posts on what I called the "most important graph in astrophysics." Today, I want to finish the series by explaining how the HR diagram shows us how stars age and evolve.

Stellar evolution: a star's life cycle

You can read the first and second posts here and here, respectively. But for completeness, let's restate that the HR diagram is a plot with stellar luminosity (L for energy output) on the vertical axis and stellar surface temperature (T for temperature) on the horizonal axis. In the previous posts, we learned that when you measure L and T for a bunch of stars and then drop them onto this kind of plot, you find the majority of the points fall on a thick diagonal band running from high stellar luminosity and temperature (high L and T) to low stellar luminosity and temperature (low L and T). That band is what astronomers call the Main Sequence, and its discovery in the HR diagram was key to understanding what stars were and how they shined.

What the Main Sequence revealed were stars in their long middle age. Middle-aged stars (meaning stars in between their relatively short birth and death phases) support themselves against their own crushing, titanic gravity by releasing energy through fusion reactions in their hot, dense cores. Hydrogen nuclei are fused into helium nuclei, giving up a little energy along the way through good ol' E = mc².

As long as there is hydrogen to burn in the core, a star is stable, happy, and free to shine its brilliance into the dark night of space. Luckily stars have lots of hydrogen to burn. A star like the sun contains about a billion billion billion tons of hydrogen gas. That translates into about 10 billion years of life on the Main Sequence. But a billion billion billion tons of gas is not infinite. Eventually, the hydrogen fusion party must end. The star will run out of fuel in the core, and that is when it stops being middle-aged.

How the HR diagram depicts stellar evolution

Credit: Richard Powell via Wikipedia

What happens next is also revealed by the HR diagram, which once again, is why it is the most important graph in astrophysics. When astronomers first started dropping their stars onto the diagram more than 100 years ago, they saw not only the Main Sequence but also stars clustered in other places. There were lots of moderately bright stars with low temperatures (high L and low T). There were also lots of really, really bright stars with even lower temperatures (very high L and lower T). Using the laws of physics associated with hot glowing matter, astronomers could derive the sizes of these bright cool stars and found that they were much bigger than the sun. They identified giant stars (the bright ones), which were 10 times the size of the sun, and supergiants (the really, really bright ones), which were 100 times the size of the sun.

These various kinds of giant stars on the HR diagram were the all-important evidence for the evolution of stars. Stellar properties were not static. They aged and changed just like we did. Astrophysicists eventually saw that the evolution of a star on the HR diagram was driven by the evolution of nuclear burning in its core. As researchers got better at modeling what happens within stars as they age, they came to see that after the hydrogen fuel runs out in the core, gravity begins to crush what is left: inert helium "ash."

Eventually, the gravitational squeeze drives temperatures and densities in the core high enough to ignite the helium ash, allowing the helium nuclei to fuse into carbon nuclei. These internal changes rearrange the outer layers of the star, making them swell and bloat †first into the giants, and then into the supergiants. The details of why they get so large are complicated and require lots of detailed calculations (done with computers). What matters for us is that what comes out of those calculations are evolutionary tracks across the HR diagram. The tracks are predictions, telling astronomers how changes in a star's nuclear burning history will manifest in it its luminosity and temperature which, in turn, translates into how it will move across the HR diagram over time.

The changes for actual stars are too slow to watch over a human lifetime. But by taking measurements of lots of random stars (meaning they are at random points in their evolution), we can find the older ones in their giant or supergiant phases. Then, via some statistics, astronomers can then see if their theoretical evolutionary tracks match what they see in the HR diagram. The answer is a resounding yes.

So not only do we know what stars are (big balls of mostly hydrogen gas with a fusion furnace in the core), but we also know exactly how those luminous spheres evolve across billions of years of cosmic history †including lighting up the nights for a remarkable planet that is home to some remarkable hairless monkeys.

• Before migrating to the sea, whales were terrestrial herbivores.
• As they transitioned to the ocean and became carnivores, at least one proto-whale was an eating machine †both on land and in the sea.
• This fearsome fossil was found in the Sahara desert, which used to be an ocean bottom.

Whales are carnivorous, although gigantic baleen whales feed on such tiny prey that it is hard to believe that they ever get enough food. Toothed whales dine on fish, squid, and octopi. (Fun fact: Orcas, also known as killer whales, are not really whales †they are gigantic, killer dolphins.)

Before moving into the sea, it is believed that whales were once terrestrial herbivores and somewhat deer-like. About 43 million years ago, say the authors of a new study, at least one protocetid †a semi-aquatic whale †was something else entirely. According to a fossil recently found in the Sahara desert, it was a four-legged, walking, swimming, Jurassic World-like nightmare.

In more gentle, scientific terms: "Unique features of the skull and mandible suggest a capacity for more efficient oral mechanical processing than the typical protocetid condition, thereby allowing for a strong raptorial feeding style."

Likely feeding on both land and in the sea, Phiomicetus anubis was fierce, with teeth and jaws powerful enough to tear apart its prey. The study says that it was about 10 feet long and weighed as much as 1,300 pounds, with a head reminiscent of a jackal's. Thus, its discoverers had good reason to name it after Anubis, the Egyptian god of death.

The Sahara was not always a desert

Authors Mohamed Sameh, Abdullah Gohar, and Hesham Sallam with the Phiomicetus anubis fossil.Credit: Abdullah Gohar (courtesy of the authors)

P. anubis was found in the Sahara desert, which was once under water. It is believed that such Protocetidae hail from the Eocene era in the Indo-Pakistan region. Its fossil was found in the Fayum Depression in the Egyptian Western Desert, an area that has been the site of numerous prehistoric whale discoveries, as well as discoveries of early fish, sharks, and land mammals.

The researchers did not find all of P. anubis, but they did find enough to deduce its characteristics based on its cranium, jaws, cervical and thoracic vertebrae, and its rib fragments.

Chow time

Examination of the whale's teeth suggests that it routinely digested prey that were too large to swallow whole and thus had to be torn apart. Among its likely fare: large fish, smaller cetaceans, turtles, and invertebrates such as nautiloids. As far as how it caught and killed its food, the study says:

"Phiomicetus may have used the same mechanism that modern crocodilians and sharks use in hunting large prey items (e.g. large fish or small cetaceans) by pulling them onto land or tearing them by seizing a part of the prey with powerful jaws then rolling and twisting the entire body."

The researchers also assert that P. anubis was not above scavenging.

• Conventionalism is the idea that right and wrong depend entirely on the cultures and traditions to which we belong. What one culture calls immoral, another accepts as normal.
• But if we accept this reasoning, we are forced to conclude that any manner of atrocities †from ritual child sacrifice to female genital mutilation †are permissible.
• Without a universal standard of morality, it is difficult to claim that some behaviors are always wrong.

On August 17, 2021, the Taliban, fresh from their takeover of Afghanistan, released a statement that they would protect the existing rights of Afghan women "within the framework of Islam." There was no mention of "human rights" or the Western idea of equality. Instead, the Taliban were appealing to a 2500-year-old tradition †the idea of cultural relativity.

The belief that right and wrong and good and bad depend on the culture to which one belongs is popular today. We are loath to say that one culture is better or worse than another, that one way of doing things is "immoral," or that some traditions are a bit disgusting or weird. Doing so risks being labeled culturally insensitive †or worse. But what do we lose by being willfully blind to vast differences in moral standards between cultures?

You say potato, I say ritual child sacrifice

From where do you get your morals? Your sense of right and wrong? If you are an absolutist, it will presumably come from some kind of universal (possibly religious) moral order. But if you are a relativist, you likely will point to some worldly source, like society, family, or personal conscience.

Broadly speaking, there are two kinds of relativists: subjectivists and conventionalists.

Subjectivists are those who believe values and morals are made entirely by you as an individual. It finds expression in French philosopher Jean-Paul Sartre, who wrote, "You are free, therefore choose †that is to say, invent. No rule of general morality can show you what you ought to do."

But it is found also in Frederick Nietzsche's "perspectivism," which allows for no facts at all †not least moral facts †but only interpretations from our particular vantage point. When he wrote that "every great philosophy up till now has consisted of… the confession of its originator, and a species of involuntary and unconscious autobiography," he meant to say that whenever we declare this or that to be the case, we are only expressing or projecting our own values.

So, subjectivists will argue that you have your values, and I have mine. I am a vegetarian, and you are a meat eater. You are happy to download movies illegally, and I am not. And that is okay.

Conventionalists, on the other hand, argue that our morals come from the society, culture, or historical norms of our time. They see right and wrong as embedded in the traditions and conventions of our age. And the first clear formulation of this is found in Herodotus.

It's all Greek to me

Any well traveled person can attest that the more you see of the varied and incongruous behaviors across the world, the more you see your own afresh. But in spite of all these different cultures, we have an incredibly hard time abandoning our own.

As Herodotus wrote in Histories †a sprawling, entertaining, and often hilariously inaccurate account of the peoples and times of the 5th century BCE †if someone were asked "out of all the customs in the world" which they thought were the best, they would almost always "end by preferring their own. So convinced are they that their own usages far surpass those of all others."

As wild and exciting as a new land might be, we still come back home and tell our friends how strange and weird it was.

To flesh out his point, Herodotus tells a story involving the Persian emperor, Darius. Darius asked his Greek captives if they would accept payment to eat their dead father's body. The Greeks said they would never, ever do so. The proper funeral rites of Greece demanded corpses to be burned. Darius then sent for his Indian captives, a group of people called the "Callatians", who he knew did eat their father's body after death. Would they accept any money to burn their father's bodies? They were shocked and offended. It was an utter sacrilege, and they refused any money.

His point? What one people thinks is the height of taboo, another will see as normal. We each have the morals that we do simply because we were born in a particular time and place.

Can we declare that another culture is immoral?

The problem with conventionalism, or cultural relativity, is that it is hard to see how we can ever judge another culture's questionable practices. If we truly believe that "right and wrong are wholly defined by society," then we are forced to admit that any manner of atrocities †from ritual child sacrifice to the Holocaust †are okay.

If a country has a long and popular tradition of female genital mutilation, then we have no grounds to say that it is wrong. If a culture accepts child marriages or a pre-adolescent age of consent, that is just the way they do things. And, with our opening example, if the Taliban or Afghan Muslim tradition subjugates women and forbids free speech, what grounds do we have to rebuke them?

It seems that there are few ways out of this problem. One way might be to commit to what is called "political realism," which dates back to another "father of history" in the ancient Greek, Thucydides. This theory maintains that right and wrong, or the idealistic notions of justice and honor, simply do not apply in international relations. When we are dealing with states against states, it is dog-eat-dog or "might makes right" †a form of ethical survival of the fittest, perhaps.

But this is unsatisfactory. If we are to salvage the idea that certain beliefs or practices are morally wrong, reprehensible even, no matter where or when they occur, we likely have to revert to some kind of moral absolutism. This is no easy position to argue, since to do so seems to require some universal or objective ethical yardstick by which to bash others. We need a reply when someone asks, "On what grounds is your way better?" One way, of course, is religion. But if not religion, then what?

It is a thorny issue, but what the Taliban's wily and manipulative press conference has revealed is that there is a paucity to moral relativism that most find hard to accept. Perhaps Afghanistan has taught us that we are more absolutist than we thought.

Jonny Thomson teaches philosophy in Oxford. He runs a popular Instagram account called Mini Philosophy (@philosophyminis). His first book is Mini Philosophy: A Small Book of Big Ideas.

In recent decades the cost of wind and solar power generation has dropped dramatically.

This is one reason that the U.S. Department of Energy projects that renewable energy will be the
fastest-growing U.S. energy source through 2050.

However, it's still relatively expensive to store energy. And since renewable energy generation
isn't available all the time – it happens when the wind blows or the sun shines – storage is essential.

As a
researcher at the National Renewable Energy Laboratory, I work with the federal government and private industry to develop renewable energy storage technologies. In a recent report, researchers at NREL estimated that the potential exists to increase U.S. renewable energy storage capacity by as much as 3,000% percent by 2050.

Here are three emerging technologies that could help make this happen.

Longer charges

From alkaline batteries for small electronics to lithium-ion batteries for cars and laptops, most people already use batteries in many aspects of their daily lives. But there is still lots of room for growth.

For example, high-capacity batteries with long discharge times – up to 10 hours – could be valuable for storing solar power at night or increasing the range of electric vehicles. Right now there are very few such batteries in use. However, according to
recent projections, upwards of 100 gigawatts' worth of these batteries will likely be installed by 2050. For comparison, that's 50 times the generating capacity of Hoover Dam. This could have a major impact on the viability of renewable energy.

Batteries work by creating a chemical reaction that produces a flow of electrical current.

One of the biggest obstacles is limited supplies of lithium and cobalt, which currently are essential for making lightweight, powerful batteries. According to
some estimates, around 10% of the world's lithium and nearly all of the world's cobalt reserves will be depleted by 2050.

Furthermore, nearly 70% of the world's cobalt is mined in the Congo, under conditions that have long been documented as
inhumane.

Scientists are working to develop techniques for
recycling lithium and cobalt batteries, and to design batteries based on other materials. Tesla plans to produce cobalt-free batteries within the next few years. Others aim to replace lithium with sodium, which has properties very similar to lithium's but is much more abundant.

Safer batteries

Another priority is to make batteries safer. One area for improvement is electrolytes – the medium, often liquid, that
allows an electric charge to flow from the battery's anode, or negative terminal, to the cathode, or positive terminal.

When a battery is in use, charged particles in the electrolyte move around to balance out the charge of the electricity flowing out of the battery. Electrolytes often contain flammable materials. If they leak, the battery can overheat and catch fire or melt.

Scientists are developing solid electrolytes, which would make batteries more robust. It is much harder for particles to move around through solids than through liquids, but
encouraging lab-scale results suggest that these batteries could be ready for use in electric vehicles in the coming years, with target dates for commercialization as early as 2026.

While solid-state batteries would be well suited for consumer electronics and electric vehicles, for large-scale energy storage, scientists are pursuing all-liquid designs called
flow batteries.

A typical flow battery consists of two tanks of liquids that are pumped past a membrane held between two electrodes. (
Qi and Koenig, 2017, CC BY)

In these devices both the electrolyte and the electrodes are liquids. This allows for super-fast charging and makes it easy to make really big batteries. Currently these systems are very expensive, but research continues to
bring down the price.

Storing sunlight as heat

Other renewable energy storage solutions cost less than batteries in some cases. For example,
concentrated solar power plants use mirrors to concentrate sunlight, which heats up hundreds or thousands of tons of salt until it melts. This molten salt then is used to drive an electric generator, much as coal or nuclear power is used to heat steam and drive a generator in traditional plants.

These heated materials can also be stored to produce electricity when it is cloudy, or even at night. This approach allows concentrated solar power to work around the clock.

Checking a molten salt valve for corrosion at Sandia's Molten Salt Test Loop. (
Randy Montoya, Sandia Labs/Flickr, CC BY-NC-ND)

This idea could be adapted for use with nonsolar power generation technologies. For example, electricity made with wind power could be used to heat salt for use later when it isn't windy.

Concentrating solar power is still relatively expensive. To compete with other forms of energy generation and storage, it needs to become more efficient. One way to achieve this is to increase the temperature the salt is heated to, enabling more efficient electricity production. Unfortunately, the salts currently in use aren't stable at high temperatures. Researchers are working to develop new salts or other materials that can withstand temperatures as high as 1,300 degrees Fahrenheit (705 C).

One leading idea for how to reach higher temperature involves heating up sand instead of salt, which can withstand the higher temperature. The sand would then be moved with conveyor belts from the heating point to storage. The Department of Energy recently announced funding for a
pilot concentrated solar power plant based on this concept.

Advanced renewable fuels

Batteries are useful for short-term energy storage, and concentrated solar power plants could help stabilize the electric grid. However, utilities also need to store a lot of energy for indefinite amounts of time. This is a role for renewable fuels like
hydrogen and ammonia. Utilities would store energy in these fuels by producing them with surplus power, when wind turbines and solar panels are generating more electricity than the utilities' customers need.

Hydrogen and ammonia contain more energy per pound than batteries, so they work where batteries don't. For example, they could be used
for shipping heavy loads and running heavy equipment, and for rocket fuel.

Today these fuels are mostly made from natural gas or other nonrenewable
fossil fuels via extremely inefficient reactions. While we think of it as a green fuel, most hydrogen gas today is made from natural gas.

Scientists are looking for ways to produce hydrogen and other fuels using renewable electricity. For example, it is possible to make hydrogen fuel by
splitting water molecules using electricity. The key challenge is optimizing the process to make it efficient and economical. The potential payoff is enormous: inexhaustible, completely renewable energy.

Kerry Rippy, Researcher, National Renewable Energy Laboratory

This article is republished from
The Conversation under a Creative Commons license. Read the original article.