What is a star? We are all pretty familiar with stars. We see them on most clear nights as tiny shining light spots of light in the sky. Stars are the topic of countless poems, stories, and children’s songs. But, exactly, what is a star?

A star is a ball of gas, which gives out light. The gas is mostly made up of H₂ and He, held together by its own gravity. Nuclear fusion(核聚变)reactions in its center part support the star against gravity and produce photons(光子)and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth.

According to present theory, stars are born as a huge ball within huge gas clouds that fall in suddenly themselves. The cloud’s material heats up as it falls inward under the force of its own gravity. When the gas reaches about 10 million K(18 million℉), H₂ nucleus(原子核)begin to fuse into He nucleus, and the star is born. Energy from nuclear fusion radiates outward from the center of the rapid-growing star, and gradually prevents the gas cloud from falling in.

A star’s color relies on its temperature: hotter stars give out bluer light and cooler stars give out redder light. Temperature is also related to weight. Red dwarf stars have as little as 0.075 solar and the surface temperature is less than 4,000 K. The heaviest star known at present is R136a1, a Wolf-Rayet star 265 of times the Sun—its surface temperature keeps at a red-hot 50,000 K.

The heaviest and hottest stars exhaust their energy within a few million years, while tiny and cool red dwarf stars can keep on burning for many billions of years.

Researchers in the field of psychology have found that one of the best ways to make an important decision, such as choosing a university to attend or a business to invest in, involves the utilization of a decision worksheet. Psychologists who study optimization compare actual decisions made by people with theoretical ideal decisions to see how similar they are. Supporters of the worksheet procedure believe that it will yield optimal, that is, the best decisions. Although there are several variations on the exact format that worksheet can take, they are all similar in their essential aspects. Worksheets require defining the problem in a clear and concise way and then listing all possible solutions to the problem. Next, the relevant considerations that will be affected by each decision are listed, and the relative importance of each consideration or consequence is determined. Each consideration is assigned a numerical value to reflect its relative importance. A decision is mathematically calculated by adding these values together. The alternative with the highest number of points emerges as the best decision.

Since most important problems are multifaceted (多层面的), there are several alternatives to choose from, each with unique advantages and disadvantages. One of the benefits of a pencil and paper decision-making procedure is that it permits people to deal with more variables than their minds can generally comprehend and remember. On the average, people can keep about seven ideas in their minds at once. A worksheet can be especially useful when the decision involves a large number of variables with complex relationships. A realistic example for many college students is the question “What will I do after graduation?” A graduate might seek a position that offers specialized training, pursue an advanced degree, or travel abroad for a year.

A decision-making worksheet begins with a brief statement of the problem that will also help to narrow it. It is important to be clear about the distinction between long-range and immediate goals because long-range goals often involve a different decision than short-range ones. Focusing on long range goals, a graduating student might revise the question above to “What will I do after graduation that will lead to a successful career?”

Would you save a woman, a child or a dog in a car crash? An experiment has investigated human morality（道德观）and ranked countries based on who they would save in the event of a certain death situation.

The findings show the value of life varies according to different countries. French people, for example, are far more likely to save women than men. Lithuania ranks in the top 10 for saving both the young and the healthy. The four characters most people saved in the experiment are a baby, a little girl, a little boy and a pregnant woman.

The experiment put forward difficult ethical（伦理的）decisions such as choosing between the lives of a family of four crossing the road and a group of old people going the other way. Quandaries（两难境地）like this will one day be faced by autonomous vehicles that will be programmed with rules that place value on human life.

Tests into the moral decisions involving almost forty million people around the world, including the English, have found that the lives of women and men are widely held in equal regard. The French however resist the trend as they think the lives of women have far more weight than those of men.

Dr Edmond Awad, of the MIT Media Lab, who is the lead author, said, “The study is basically trying to understand the kinds of moral decisions that driverless cars might have to turn to. We don't know yet how they should do that.” He added, “we find that there are three elements that people seem to approve of the most.”

“These are that humans are saved over animals, that many people are saved rather than a few, and that the young are saved rather than the old.”

Dr Awad added, “The main preferences are to some degree universally agreed upon. But the degree to which they agree with this varies among different groups or countries.”