i need help with triggers

in my game you fly a ship and need to avoid hitting objects
ive set up objects to move towards the ship but what i want to make is a sphere trigger on the ship that activates the object movement so they dont start moving towards the ship as the game starts but rather when it triggers the sphere

void OnTrigger enter () dosent work i think because the code needs to be in void update

anyways heres the code:

using UnityEngine;
using System.Collections;

public class ObjectAi : MonoBehaviour {
public Transform target;
public int objectSpeed;
public Transform Ship;
public int rotationSpeed;

private Transform myTransform;

void Awake() {
	myTransform = transform;

// Use this for initialization
void Start () {
	GameObject go = GameObject.FindGameObjectWithTag("Ship");
	    target = go.transform;


// Update is called once per frame
void Update () {

	Debug.DrawLine(target.transform.position, myTransform.position);
//look at target
	myTransform.rotation = Quaternion.Slerp(myTransform.rotation, Quaternion.LookRotation(target.position - myTransform.position), rotationSpeed * Time.deltaTime);
   // move towards target
	myTransform.position += myTransform.forward * objectSpeed * Time.deltaTime;	

void OnTriggerEnter(Collider other)


Use OnTriggerEnter to set a boolean variable to ‘true’ and then use an if statement in Update, testing that variable, to make the objects move towards the player.

thanks worked.