The Scientific Method

Look up in the sky. How many stars do you see? Do they all look the same? How long have they been there? What are stars made of?

Mauna Kea Observatory, Hawaii

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What do you know about stars?Stars are born, shine brightly for millions or billions of years, then collapse and even explode. How do you think these changes happen? How do scientists study stars?

Orsola De Marco and Mordecai-Mark Mac Low are astrophysicists. They both work at the American Museum of Natural History in New York City. Astrophysicists are scientists who are curious about how the universe works.

Astrophysicists use different methods to gather data. For example, Orsola studies light to investigate the history of the universe and the behavior of objects in space.

Through a telescope, Orsola observes the light given off by distant stars. However, the length of time she can observe does not permit her to see events that may take many, many years to finish. Mordecai uses computer models to investigate how the universe works. Mordecai enters data into a computer. The data he enters are processed by the computer to make a model of an event in space. The model can show what happens in distant space over time.

By working together, their different skills increase our understanding of stars. What do scientists like Mordecai and Orsola learn about stars from their different methods of work?

Orsola studies stars by

looking through a telescope.

Mordecai studies stars by making computer models.

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What do scientists do?Astrophysicists know that, over billions

of years, stars change. They call this the “life cycle” of stars. Stars form, mature, and eventually die. Orsola and Mordecai use the scientific method when they study this process. They and other scientists have observed large numbers of stars, formed hypotheses about stars, and tested these hypotheses with more observations.

The scientific method is used by all kinds of scientists to investigate and answer questions. This method helps them explain natural phenomena. It also makes it possible for others to reliably repeat their procedures. This way, the work can be independently checked. Scientists do not always follow all the steps of the scientific method in order. However, they always keep careful records of what they have done and observed.

Astrophysicists have observed that some stars have a companion star. The two stars

A disk of gas and dust around a young star may form into planets.

Aging Sunlike stars cool and expand, becoming red giants.

Life Cycle of a Star

4EXPLAIN

orbit around each other, and are called a binary. In some binaries, the two stars can be a billion kilometers apart. In others, the two stars orbit so tightly that it is difficult to see that they are separate.

Orsola and Mordecai have been studying binaries for years. Sometimes Orsola observes tightly-orbiting binaries where one of the stars is a white dwarf. The distance between them is actually less than the size of the original red giant that became the white dwarf. Why does this happen?

Astrophysicists have been investigating this question for some time. They have a hypothesis, a statement they can test to answer a question. Their hypothesis is that the red giant “ate” its companion star.

The companion star spiraled in toward the core of the giant, driving away the giant’s outer gas layers. The red giant became a white dwarf, and the companion star ended up in a tight orbit.

Forming a Hypothesis

Ask lots of “why” questions.

Look for connections between

important variables.

Suggest possible explanations

for these connections.

▶ Make sure the explanations

can be tested.

They die as white dwarfs—Earth-size stars with half the mass of the Sun.

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How do scientists test

their hypothesis?Orsola and Mordecai want to

test this hypothesis. To do this, they need to collect more data. Orsola spends weeks at a telescope. She measures distances between the two stars in binaries by observing how they move. She watches and records how long the two stars of a binary take to orbit each other. She uses this information to calculate the distances between the stars, and combines her data with data reported by other scientists.

Binary stars take five to ten years to form. This seems like a long time, but it is incredibly fast by astronomy standards. However, the universe is so vast that a scientist could spend a lifetime at a telescope and never find a star eating its companion. So, Orsola and Mordecai have to use a computer model to test their hypothesis. Mordecai’s model can then be compared to Orsola’s observations.

▲ Orsola looks at the model to compare with her data gathered from a telescope.

◀ interior of Kitt Peak Observatory

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A model is a computer program that shows natural processes at work. “I need a model that uses the laws of physics to predict the orbits of the stars after they merge,” Mordecai explains. “Fortunately, fundamental processes such as gravity and pressure do not only apply on Earth. They apply across the universe.” These forces are an important part of the model. Mordecai also enters the initial values for the key variables of his model. For example, the masses of the stars that make up the binary and the distance between them. He runs the model many times, changing the initial values for the variables each time.

Testing a Hypothesis

Think about the different kinds

of data that could be used to

test the hypothesis.

Choose the best method

to collect this data:

• Perform an experiment

(in the lab)

• Observe the natural world

(in the field)

• Make a model

(on a computer)

Plan a procedure and gather data.

▶ Make sure the procedure can

be repeated.

Kitt Peak Observatory, located near Tucson, Arizona

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How do scientists analyze data?Each run of the model takes approximately a week

on a supercomputer to finish. Each run predicts a final distance between the two stars. After all the models are run, Mordecai has a range of outcomes that reflect the different sets of initial values for the key variables (mass and distance between two stars prior to run).

▲ This is a series of still images taken from one of Orsola and Mordecai’s visualizations.

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Part of testing a hypothesis is arranging the data to look for patterns. Orsola and Mordecai organize their data so they can compare Mordecai’s predictions to Orsola’s observations.

Mordecai does this by creating visualizations of the data that his model generated. Visualizations are images or movies that represent complex sets of data. Visualizations of Mordecai’s runs show what happens as stars of different masses and initial distances swirl into the center of a red giant.

Mordecai compares his computer model runs.

Analyzing the Data

Organize the data as a chart

such as a table, graph, diagram,

map or group of pictures.

Look for patterns in the chart

that show connections between

important variables in the

hypothesis being tested.

▶ Make sure to check the data by

comparing it to data from other

sources.

9EXPLAIN

How do scientists

draw conclusions?

Now it is time for Orsola and Mordecai to compare the model predictions to the observations. They compare the distances between the two stars in binaries produced by the model to the distances that Orsola and other astrophysicists observed in space. If the results from the model and the observations agree, that is evidence in favor of the hypothesis. If the results disagree, either the hypothesis is flawed or the model is incomplete. “So far, we have found some preliminary predictions in the model that match observed data!” says Orsola, “which is very exciting.”

However, some of the models also predict that when a red giant eats a companion star, the two stars combine into one. “The model shows us that the process can produce another kind of object in the universe: a single star formed when two stars combine,” she explains. “This is really fascinating!”

Mordecai and Orsola discuss their conclusions. ▶

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Orsola and Mordecai’s results lead them to ask more questions. Can they find stars that are evidence of this second outcome? What can Orsola learn from Mordecai’s model about where and how to look for these stars? New questions, like these, are important because they lead to new hypotheses and new discoveries.

The Great Nebula in the constellation Orion has many stellar nurseries where stars and solar systems are forming. On a clear, dark winter night, the nebula can be seen as a fuzzy patch in Orion’s sword.

Drawing Conclusions

Decide if the data clearly support

or do not support the hypothesis.

If the results are not clear rethink

how the hypothesis was tested

and make a new plan.

Write up the results to share

with others.

▶ Make sure to ask new questions.

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Scientists use many skills as they work through the scientific method. Skills help them gather information and answer questions they have about the world around us. Here are some skills they use:

▲ A Punnett square is used to predict the possible outcomes of a genetic cross.

▲ Use diagrams and drawings to help classify living and nonliving things.

Observe Use your senses to learn about an object or event.

Form a Hypothesis Make a statement that can be tested to answer a question.

Communicate Share information with others.

Classify Place things with similar properties into groups.

Use Numbers Order, count, add, subtract, multiply, or divide to explain data.

Make a Model Make something to represent an object or event.

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Use Variables Identify things that can control or change the outcome of an experiment.

Interpret Data Use information that has been gathered to answer questions or solve a problem.

Measure Find the size, distance, time, volume, area, mass, weight, or temperature of an object or event.

Predict State possible results of an event or experiment.

Infer Form an idea or opinion from facts or observations.

Experiment Perform a test to support or disprove a hypothesis.

In each chapter of this book, you will find an Inquiry Skill Builder. These features will help you build the skills you need to be a scientist.

▲ Use a table to help organize and interpret data.

◀ Use a calculator to perform long or complex calculations or to verify your work.

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In the Classroom

• Go with a trusted adult—such as your teacher, or a parent or guardian.

• Do not touch animals or plants without an adult’s approval. The animal might bite. The plant might be poison ivy or another dangerous plant.

In the Field

ResponsibilityTreat living things, the environment, and one another with respect.

• Read all of the directions. Make sure you understand them. When you see “ Be Careful,” follow the safety rules.

• Listen to your teacher for safety directions. If you do not understand, ask for help.

• Wash your hands with soap and water before an activity.

• Be careful around a hot plate. Know when it is on and when it is off. Remember that the plate stays hot for a few minutes after it is turned off.

• Wear a safety apron if you work with anything messy or that might spill.

• Clean up a spill right away, or ask your teacher for help.

• Tell your teacher if something breaks. If glass breaks, do not clean it up yourself.

• Do not eat or drink anything during an experiment.

• Wear safety goggles when your teacher tells you to wear them. Wear them when working with anything that can fly into your eyes or when working with liquids.

• Be careful around a hot plate. Know when it is on and when it is off. Remember that the plate stays hot for a few minutes after it is turned off.

• Keep your hair and clothes away from open flames. Tie back long hair, and roll up long sleeves.

• Keep your hands dry around electrical equipment.

• Put equipment back the way your teacher tells you to.

• Clean up your work area after an activity, and wash your hands with soap and water.

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