To prevent blood flowing back after each heartbeat. They ensure that the blood flows only in one direction (back to the heart).
In genetics, gene expression is the most fundamental level at which the genotype gives rise to the phenotype, i.e. observable trait. ... Such phenotypes are often expressed by the synthesis of proteins that control the organism's structure and development, or that act as enzymes catalyzing specific metabolic pathways.
Answer:
is about 4.6 billion years old
The Hertzsprung-Russell diagram is one of the most important tools in the study of stellar evolution. Developed independently in the early 1900's by Ejnar Hertzsprung and Henry Norris Russell, it plots the temperature of stars against their luminosity (the theoretical HR diagram), or the color of stars against their absolute magnitude
Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR diagram as it evolves. This reveals the true power of the HR diagram – astronomers can know a star’s internal structure and evolutionary stage simply by determining its position in the diagram.
The Hertzsprung-Russell diagram the various stages of stellar evolution. By far the most prominent feature is the main sequence (grey), which runs from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) of the diagram. The giant branch and supergiant stars lie above the main sequence, and white dwarfs are found below it.
