How Vast is the Universe and How Far Are the Stars from Us? These questions may be among the oldest and most fundamental inquiries of humanity, and they are also among those that scientists are actively exploring and attempting to answer.
However, answering these questions is not easy. The universe is so vast and complex that we cannot directly observe and measure it.
So how do humans know the size and distance of the universe? How can we calculate the positions and characteristics of galaxies and nebulae that are tens of billions of light-years away?
To answer this question, we need to understand an important concept: the cosmic distance ladder.
The universe is too vast for us to measure directly. (Image source: Astronomy).
This is a series of methods and tools developed by scientists to measure the positions and sizes of celestial bodies across different distance ranges. These methods and tools function like rungs of a ladder, allowing us to gradually explore deeper cosmic spaces.
Why do we need such a ladder? Because celestial bodies at different distance ranges have different characteristics and patterns, so they need to be measured using different methods.
For example, for the inner planets of the Solar System, we can use radio waves to measure the distance between them and Earth; for stars that are farther away, we can use the method of trigonometric parallax to measure the angle between them and Earth. For distant galaxies, we can use supernovae or Hubble’s Law to measure the redshift between them and Earth. Each step taken relies on the measurements obtained from the previous step, thereby establishing a consistent and reliable system. So what are the specific methods used to measure the universe?
(Image source: Astronomy).
Radio Reflection Method
The radio reflection method is a measurement method that uses radio waves transmitted to the surface of a nearby planet and receives the reflected signal; the distance is calculated by measuring the time it takes for the signal to return.
This method is suitable for celestial bodies within the Solar System and was first used in the 1950s. At that time, American scientists used military radar systems to transmit radio waves to the Moon and received reflected signals a few seconds later.
This method was subsequently used to measure the distances between Earth and other planets, such as Venus and Mars, as well as asteroids and comets. The advantage of the radio reflection method is its extremely high accuracy, allowing for measurements with millimeter-level precision.
This enables scientists to better understand the distances and relative positions of planets. However, the radio reflection method also has some limitations. It only works on solid-surfaced objects and does not operate effectively on objects with thick atmospheres or clouds, such as Jupiter and Saturn. This is because radio waves are scattered or absorbed in the atmosphere, making it impossible to accurately measure the time of the reflected signal.
This method is suitable for celestial bodies within the Solar System. (Image source: Astronomy).
Trigonometric Parallax Method
The trigonometric parallax method is an ancient and intuitive measurement method that uses the positional change of stars in the sky as Earth orbits the Sun to calculate the angles between the stars and Earth, thereby inferring their distances.
This method is effective for stars relatively close to Earth, typically within a distance of 100 light-years. Based on the principles of the trigonometric parallax method, we can use triangles in geometric relationships to solve for the lengths of unknown sides. Here, two known sides are the baseline formed by Earth as it orbits the Sun, approximately 300 million km. One known angle is the positional change of a star in the sky in relation to the Sun, known as the parallax angle.
This method is effective for stars relatively close to Earth. (Image source: Astronomy).
In the early 19th century, German astronomer Friedrich Bissell successfully applied the trigonometric parallax method for the first time to measure the parallax angle of a star named 61 Pegasus, calculating its distance from Earth to be 10.4 light-years. This breakthrough made the trigonometric parallax method an important means of measuring distances to stars.
Subsequently, scientists used this method to measure the distances of many more stars, including the closest star to Earth, Alpha Centauri, which is 4.3 light-years away.
- The advantage of this method is that it can be calculated directly and simply, without relying on assumptions or other models.
- However, it also has a disadvantage in that it only works for stars close to Earth. For stars farther away, the parallax angle becomes very small and difficult to measure accurately.
(Image source: Astronomy).
Measuring Distance Using Hubble’s Law
Hubble’s Law is an important law that describes the expansion of the universe; it measures the distance between distant objects and us by observing their redshift values.
This method can be applied to very distant galaxies and even the observable universe’s event horizon, which is approximately 10 billion light-years or more. Hubble’s Law was discovered and proposed by American astronomer Edwin Hubble in the 1920s. He observed many extragalactic galaxies and discovered an astonishing fact: these galaxies are all moving away from us at different speeds, and the farther they are, the faster they are moving away.
This means that the universe is not static; rather, it is continuously expanding and evolving. Hubble’s Law can be expressed by a simple formula: v = H0d. In this formula, v represents the speed of the galaxy moving away from us, d represents the distance between the galaxy and us, and H0 is the Hubble constant, which describes the expansion rate of the universe. By measuring the redshift of galaxies, we can determine how fast they are moving away from us.
Hubble’s Law. (Image source: Astronomy).
Then, using Hubble’s Law, we can calculate the distances of distant galaxies from us. The beauty of Hubble’s Law is that it can be used to measure extremely far distances that are typically impossible to measure. Furthermore, Hubble’s Law is based on the properties of the entire universe rather than the characteristics of individual celestial bodies. This allows us to study the broader scale of the universe and understand its evolution and structure.
However, Hubble’s Law also has some disadvantages.
- First, it relies on the accuracy and stability of the Hubble constant. This constant may change over time and space.
- Second, factors such as the rapid expansion of the universe, dark energy, and dark matter will also affect Hubble’s Law.
Therefore, when performing measurements using Hubble’s Law, we need to take these factors into account and make appropriate adjustments. Although Hubble’s Law has some limitations and challenges, it remains one of the most important tools in helping us understand the universe. By continuously improving observational technology and accurately measuring the Hubble constant, we can delve deeper into the mysteries of the universe and unveil its origins, evolution, and future destiny.
